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(Online) ISSN 2744-1741 
Defense and Security Studies  Original Research 
Vol. 6, No. 2, 2025, pp.122-131 
https://doi.org/10.37868/dss.v6.id295 

This work is licensed under a Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/) that allows others 
to share and adapt the material for any purpose (even commercially), in any medium with an acknowledgement of the work's authorship 
and initial publication in this journal. 

 122 

 
 
Vulnerability analysis of critical infrastructures with the Event Tree 
Technique   
 
Francesco Trinchillo1* 
1 Independent researcher, Naples, Italy  
 

*Corresponding author E-mail:  francesco.trinchillo.1987@gmail.com  

Received: Nov. 4, 2025 
Revised: Nov. 20, 2025 
Accepted: Nov. 26, 2025 
Online: Nov. 26, 2025 
 

Abstract 

The vulnerability analysis of a critical infrastructure is certainly a complex and 
articulated study; therefore, it requires the use of reliable decision support tools. In 
this article, after a brief review of the state of the art on risk analysis studies for 
critical infrastructures, a vulnerability analysis will be performed on a railway 
infrastructure under the hypothesis of an intentional attack by a criminal group. 
Specifically, the aim of this paper is to show how the application of the event tree 
technique - derived from the more well-known fault tree analysis - can allow to 
define the different scenarios. For each one, a probabilistic assessment will be 
carried out through numerical simulations with the Monte Carlo method in order 
to calculate the security level of this infrastructure. 

 

© The Author 2025. 
Published by ARDA. 

Keywords: Critical infrastructure, vulnerability analysis, defense of the country, 
event tree technique, Monte Carlo method, railway system 

1. Introduction  

A critical infrastructure – according to the current European legislation – is defined as an element, a plant, a 
piece of equipment, a network, a system, or a part thereof necessary for the provision of an essential service. 
Thus, protecting national critical infrastructures is necessary to guarantee the continuity of essential services, 
i.e., those that are fundamental for the maintenance of vital functions of society, economic activities, public 
health and safety, or the environment. The operation of critical infrastructures can be disrupted in the most 
varied ways that can range from a small inconvenience to total destruction; a natural catastrophic event (e.g., 
earthquakes, tsunamis, floods) or an indirect anthropogenic event (e.g., landslide, blackout), or a 
terrorist/criminal attack. 

The study of the vulnerability of critical infrastructures starts from their identification in order to define an 
appropriate level of protection that guarantees, for each typology, not only its conservation but also the 
maintenance of the supply of essential services, limiting the inconvenience for the population involved. 

The context of the operation of critical infrastructures has seen, during the years, a regulatory evolution [1]; 
now the critical infrastructures can be divided into 11 sectors: 

1. Energy 
2. Transport 



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123 

3. Banking 
4. Financial market infrastructure 
5. Health 
6. Drinking water 
7. Waste water 
8. Digital infrastructure 
9. Public administration 
10. Space 
11. Production, processing, and distribution of food. 

It is clear, therefore, that the railway system of a European Union Country is classified as a critical infrastructure; 
indeed, also the first forms of regulation of the European Commission drawn up at the beginning of the 2000s 
already include transport systems (roads, railways, ports, and airports) in the macro-categories of national 
critical infrastructures [1]. 

In the literature, there are several studies dedicated to the risk and vulnerability analysis of critical 
infrastructures, but only a few of these refer to the transport sector and specifically to railway systems. An 
interesting article [2] provides a complete and updated overview of the scientific research conducted, from 2008 
to 2019, on the topic of railway resilience, showing a constant increase in the number of articles published in 
recent years. Railway infrastructure is made up of many interconnected and interdependent elements; 
understandably, since in practice it is not possible to protect all elements of the system, the approach often 
chosen is to protect key elements. Indeed, extensive research [3] aims to explore theoretical approaches to rail 
system vulnerability: identifying key elements can guide administrators in contributing to increasing resistance 
and in appropriately developing investments for the management and protection of railway system 
infrastructure. 

Natural disasters, operational accidents, and terrorist attacks pose growing threats to railway systems, and their 
vulnerability and resilience have become major concerns for researchers and practitioners around the world. 
Thus, the importance of transport resilience and the related concepts of risk and vulnerability are also addressed 
from the perspective of shocks that transport systems may experience, which can be internal or external, natural 
or man-made, intentional or involuntary [4]. In this context, a very interesting article 6 focuses on the resilience 
of rail freight operations in the event of extreme weather events. Drawing on a British case study of rail network 
disruption following the closure of a key line in early 2016, the analysis considers the impacts on rail freight 
service provision and broader supply chains; the information provided is used to formulate a series of 
recommendations for the national rail sector. A recent study [5,6] analyzes the vulnerability and resilience of 
railway systems to disruptions from a conceptual, methodological, and practical perspective. It presents 
methodologies for assessing railway system vulnerability and resilience based on accessibility theory and 
examines the case study of the railway systems of two Chinese cities, Shanghai and Shenzhen, using the 
proposed methodologies.  Another research [7] considers the emergency management of the transport of 
dangerous goods in the event of a terrorist attack by analyzing the evolution of the accident scenario using 
advanced georeferencing tools. 

2. Vulnerability analysis of the case study 

This article will conduct a risk analysis of the vulnerability of a critical railway infrastructure, starting from an 
event similar to a terrorist attack deliberately carried out with the aim of causing serious consequences to the 
infrastructure in question. This analysis is part of the broader discipline known as Security Risk, which studies 
the analysis of risks related to human actions aimed at producing catastrophic damage; one of the first studies 
on this topic [8] extends risk analysis methodologies to such incident scenarios, offering a methodology for the 
quantitative assessment of terrorism-related risks with the aim of supporting the decision-making process. 



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124 

An interesting study [9] focused - very clearly - on the analysis of business impacts and the quantification of 
different risks, in order to define the acceptable risk level/threshold. Moreover, a recent research [10] has already 
considered similar critical infrastructures by adopting a time-based dependency risk analysis methodology, 
integrating and evaluating the effect of recovery controls. In that case, the proposed analysis methodology, 
based on awareness-based restoration, was integrated with a decision support system that enables the prediction 
of risk due to natural events to identify vulnerable and disrupted assets (e.g., electrical substations, 
telecommunications components). 

Risk analysis methods are numerous and can be divided into two macro categories: qualitative and quantitative 
[11]; to determine the logic of attack sequences and consequently the reactions of security services/systems, 
more than one method can be used; among these, an inductive method called Event Tree Analysis (ETA) has 
been chosen, a methodology derived from the more well-known fault tree analysis technique, widely used in 
maintenance engineering [12]. The event tree technique can certainly be applied to intentional attacks (terrorist 
attacks, demonstrations, industrial sabotage, etc.); in this case, the event tree's final event is the achievement of 
a specific objective (damage, destruction, disruption of service, etc.), while the events that define the tree's 
structure are sabotage actions or actions by security personnel/police; each event is associated with a duration 
and probability of occurrence. 

This article aims to carry out an analysis that shows the possible vulnerabilities of a critical infrastructure, 
hypothesizing the various attack scenarios and quantifying in probabilistic terms the risk of their occurrence. 
Consequently, useful insights can be gleaned into the "easiest" scenarios for attackers, thus defining the actual 
vulnerability level of the analyzed critical infrastructure. Therefore, based on the results obtained, it will be 
possible to determine how to improve the security level, perhaps by introducing new systems designed to thwart 
attackers' actions or, at least, react in a timely manner and thus reduce the damage. 

2.1. Railway infrastructure 

The critical infrastructure under consideration is a railway electrical substation, a "node" connected to a high-
voltage electrical grid capable of transforming and converting high voltage into a form suitable for powering 
train drives and motors. It is clear that the decommissioning of a substation will result in the failure of a section 
or even an entire railway line, resulting in the disruption of one of the country's essential services. Furthermore, 
if the affected line is a high-speed/high-capacity line, the disruptions can be truly impactful for the Country. 

Typically, a railway substation is located immediately outside a residential area and - in this case - it is assumed 
to be within an area of approximately 2000 m2, which can be accessed through an automatic sliding steel gate 
overlooking a main road. The gate leads to a yard containing a building containing electronic equipment that 
controls the electrical energy conversion systems. Outside, adjacent to the building, are the conversion and 
transformation systems, as shown in Figure 1.  

 

Figure 1. Example of a railway electrical substation 



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125 

The area has a polygonal shape and is bordered on two sides by thick, tall vegetation that prevents the passage 
of vehicles and/or people; on the other sides, however, there are 3-meter-high metal fences topped with hooked 
gates. The area is equipped with a CCTV system consisting of cameras, some fixed and some pan/tilt, positioned 
around the yard's perimeter and at the single entry/exit point, as well as those that monitor the entrances to the 
building housing the electronic equipment. The cameras are oriented so that the same point can be framed by 
multiple devices, and all are connected to the control room via a fiber optic backbone. The control room is 
located at the security department of the company that manages the country's railway network, while the nearest 
police patrol station is approximately 4 km from the substation. 

2.2. Attack scenarios and event tree construction 

This study hypothesizes an intentional attack by a criminal group aimed at damaging the electrical substation 
systems that power the transmission line for high-speed trains (Tables 1 and 2). It is assumed that the group is 
already aware of the types of defense systems (active and passive) present and, specifically, knows the exact 
point to access to interrupt the fiber optic link that powers the CCTV system. The possible scenarios envisioned 
for the case study in question all involve two sequences: one for the actual attack process, consisting of the 
criminal activities undertaken by the attackers; the second, however, is the sequence represented by the defense 
actions implemented first by the company security service and then by the police. 

Both sequences, which then trigger the various scenarios, begin with the cutting of the fiber optic backbone, 
which causes the instantaneous deactivation of the CCTV system; therefore, this action can be defined as the 
top event of the event tree, meaning it relates to an attack directed at a specific target. The duration of this action, 
although quantifiable as an average time, is deliberately excluded from the risk analysis because it is precisely 
by virtue of its identification as the top event, or initiator, from which the subsequent steps arise. In other words, 
if one were to hypothesize the introduction of the NOT operator for this event, the subsequent sequences would 
lose their meaning. 

Table 1. Average time for attack actions 

ACTIVITY AVERAGE TIME (minutes) 
Reaching the substation 3 
Entry into the substation 8 

Reaching the equipment area 5 
Sabotaging the equipment 7 

Leaving the equipment area 2 
Leaving the substation and beginning the escapement 5 

Complete escapement 5 

Table 2. Average time for defense actions 

ACTIVITY TIME (minutes) 

Security personnel interpret the situation 4 

Security personnel call the police 2 

Police action 22 

The attack sequence can be summarized as follows: 

1. Reaching the substation 
2. Entry into the substation 
3. Reaching the equipment area 
4. Sabotaging the equipment 
5. Leaving the equipment area 
6. Leaving the substation and beginning the escapement 
7. Complete escapement 



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126 

The defense sequence can be summarized as follows: 

1. Security personnel interpret the situation. 
2. Security personnel call the police. 
3. Police action. 

For each activity, whether attacking or defending, an average time expressed in minutes has been identified. 
Sequences can also be represented in the form of Gantt charts (Figs 2 and 3). 

 

Figure 2. Gantt chart with attack sequence 

 

 Figure 3. Gantt chart with defense sequence 

To better contextualize the sequences just described, it is necessary to draw up hypotheses that allow us to 
properly define the event tree and understand the infrastructure's actual vulnerability level. Specifically, we will 
assume that: 

• The CCTV system was fully functional at the time of the attack. 
• The criminal group is informed of the nature of the CCTV system and the location of the access point to the 
fiber optic backbone that powers it. 
• Cutting the backbone will certainly deactivate the CCTV system, but will also—with equal certainty—activate 
an alarm located at the company security guard post. 
• Company security cannot intervene in the event of this type of attack and must request and await the 
intervention of the police. 

The success of the attack is confirmed by the sabotage of the equipment and the complete disappearance of the 
criminal group. This final event—which, as mentioned, corresponds to the complete success of the operation—
can also be imagined as a consequence, involving the arrest of the group and, possibly, the prevention of the 
sabotage of the equipment. It is therefore assumed that police action could occur in the following phases: 

- After exiting the substation but before the group has completely disappeared (Scenario 2) 
- After the sabotaged equipment has been removed from the area, but before exiting the substation (Scenario 3) 
- After the sabotage of the equipment but before reaching the exit (Scenario 4) 
- After reaching the area where the equipment to be sabotaged is located, but before its sabotage (Scenario 5) 
- After entering the substation but before reaching the area where the equipment to be sabotaged is located 
(Scenario 6) 

It is clear that the event tree will foresee a total of 6 possible scenarios: the 5 just described in addition to the 
one that, obviously, foresees the success of the attack, which can be identified as scenario 1. 

For simplicity of discussion and based on the hypotheses made, it is assumed that reaching the substation and 
entering it (with the related break-in and tampering with the automation of the access gate) are certain actions, 
that is, that they occur with probability 1 and that the police cannot intervene before or during their execution; 
this allows us to limit the number of scenarios to those that are actually the most realistic. 

Based on the above assumptions, it is possible to construct the event tree. (Fig. 4). 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
AZIONE

Time to Action  [minutes]

Deactivation of CCTV system
Reaching the substation
Entry into the substation
Reaching the equipment area

Leaving the equipment area
Sabotaging the equipment

Leaving the substation and beginning the escapement
Complete escapement

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Time to Action  [minutes]

Security personnel call the police
Police action

Security personnel interpret the situation

AZIONE



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127 

 

Figure 4. Event tree with possible attack scenarios 

For simplicity, but without losing its meaning, it is assumed that both the corporate security service and the 
police cannot make significant errors in the execution of their tasks and that their interventions are 100% 
effective (meaning they completely block attack action) and are also detectable within an exact time frame. 
Based on this further hypothesis, while the execution times of individual actions for the criminal group's 
activities remain variable, those relating to defensive actions become exact values.  

Focusing on attack actions, the times of each of them can be considered random variables characterized by their 
own probability density distribution: for each action, therefore, the average time T already indicated in Table 1 
has been associated with a relative standard deviation σ, which, again for simplicity, is assumed equal to 30% 
of the average value. Otherwise, based on the assumption made, the times of the defense actions (Table 2) will 
be deterministic; therefore, we should not speak of mean time or standard deviation but only of action time. 

Table 3. Average time and standard deviation times for attack actions 

ACTIVITY 
AVERAGE TIME  

(minutes) 
STANDARD  
DEVIATION 

Reaching the substation 3 0,9 
Entry into the substation 8 2,4 

Reaching the equipment area 5 1,5 
Sabotaging the equipment 7 2,1 

Leaving the equipment area 2 0,6 
Leaving the substation and beginning the escapement 5 1,5 

Complete escapement 5 1,5 
 

Table 4. Exact time for defense actions 

ACTIVITY TIME (minutes) 

Security personnel interpret the situation 4 

Security personnel call the police 2 

Police action 22 

 

2.3. Probability calculation 

Having defined the six possible attack scenarios, the next step is to understand the probability of one scenario 
occurring over another, in order to determine the actual level of vulnerability of the infrastructure. The solution 
to the presented problem begins by defining the total time required to complete the generic attack sequence, 
which will undoubtedly be the sum of the completion times for each criminal activity.  

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Group Arrest and 
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SCENARIO 5
YES

Group Arrest and 
Sabotage Prevented

SCENARIO 6
YES



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Thus, indicating the total time for the generic scenario as TSi , we have: 

TSi = Ta1 + Ta2 + Ta3 +… + Tan 

 Table 5. Average time for each attack scenario 

SCENARIO 
AVERAGE TIME 

(minutes) 
SCENARIO 1 (Attack Successful) 35 

SCENARIO 2 (Group Arrest) 30 
SCENARIO 3 (Group Arrest) 25 
SCENARIO 4 (Group Arrest) 23 

SCENARIO 5 (Group Arrest and Sabotage Prevented) 16 
SCENARIO 6 (Group Arrest and Sabotage Prevented) 11 

As already mentioned, however, the times of the individual attack actions are random variables, meaning they 
can assume different values each time one approaches the resolution of the problem under analysis. They are 
characterized by their own probability density function, which, for events of this type, can be considered 
lognormal.  

Therefore, the total time of each scenario will also be a random variable whose probability density can be 
determined from the times of the individual activities.  

By applying the variable transformation method [13], it is possible to calculate the probability density associated 
with the generic scenario. The application of this methodology allows closed-form analytical solutions only for 
particular cases, that is, for those cases in which the probability densities are normal. In general, however, to 
obtain the solution, it is necessary to resort to numerical methods or Monte Carlo simulation. 

For the case study in question, the solution to the problem is obtained in two steps: the first objective is to 
calculate the probability of success of the attack (scenario 1), that is, to calculate the probability with which the 
total time required to carry out the entire attack sequence is less than the intervention time of the police: 

P attack successful = P (total time of attack sequence < time for police action) 

The second part of the problem consists in determining the probability of occurrence of all the other scenarios 
(from 2 to 6) that foresee the failure of the attack and therefore the arrest of the group due to the intervention of 
the police before the complete disappearance of the attackers; for these cases, the probability of occurrence can 
be determined as follows: 

P scenario i  = [1 - P (total time of attack sequence i < time for police action)],  for i = 2 to 6 

Applying the Monte Carlo method leads to the solution shown in Table 6. 

 Table 6. Average time and probability of occurrence for each attack scenario 

SCENARIO 
AVERAGE TIME  

(minutes) 
PROBABILITY OF  

OCCURRENCE 
SCENARIO 1 (Attack Successful) 35 0,332 

SCENARIO 2 (Group Arrest) 30 0,668 
SCENARIO 3 (Group Arrest) 25 0,357 
SCENARIO 4 (Group Arrest) 23 0,234 

SCENARIO 5 (Group Arrest and Sabotage Prevented) 16 0,153 
SCENARIO 6 (Group Arrest and Sabotage Prevented) 11 0,001 

Reporting the values in the event tree allows us to gain a more systematic view of the case study, as shown in 
Fig. 5. 



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 Figure 5. Event tree with possible attack scenarios and probability of occurrence 

3. Results Analysis 

The risk analysis conducted on this case study demonstrates how the event tree technique is able to provide a 
clear and systematic view of the problem. Specifically, it is shown that, based on the assumptions made, the 
complete success of the attack occurs with a probability of approximately 33%. This provides an objective 
measure of the vulnerability of the critical infrastructure considered. However, there are five alternative 
scenarios, all of which involve the failure of the attack with the arrest of the group, and between these two, even 
the prevention of the sabotage of the equipment. In Figure 6, the graph shows the trend in the probability of 
occurrence of each of the scenarios.  

The success of the attack (scenario 1) is not the most likely scenario; in fact, its alternative (scenario 2) - 
consisting of the complete evolution of the criminal action but characterized by police intervention immediately 
before the escape - is the most likely scenario, with a probability of occurrence around 67%. The other 
hypothesized scenarios, obviously, have a decreasing probability of occurrence; this is because the probability 
of the police intervening quickly is low. It is interesting to note that the probability of the attack's complete 
success is almost equivalent to scenario 3, which involves the group being arrested before leaving the substation; 
this highlights that, based on the protection systems in place, the final outcome is quite uncertain. 

 

Figure 6. Trend of the probability of occurrence for each scenario 

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SCENARIO
NUMBER 

SCENARIO 
PROBABILITY 

SCENARIO

YES 1 YES 1 YES 1 YES 1 YES 0,999 YES 0,847 YES 0,766 YES 0,643 YES 0,332

Attack Successful

0,001
Group Arrest and 

Sabotage Prevented
SCENARIO 6

YES 0,001

Group Arrest and 
Sabotage Prevented

SCENARIO 5 0,153
YES 0,153

Group Arrest SCENARIO 4 0,234
YES 0,234

Group Arrest SCENARIO 2 0,668
YES 0,668

Group Arrest SCENARIO 3 0,357
YES 0,357

SCENARIO 1 0,332

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4. Conclusions 

This article conducted a vulnerability analysis for a critical infrastructure within the railway system of a 
European Union Country. Specifically, it considered an electrical substation that supplies power to a high-speed 
train line. A power outage would lead to a halt in rail traffic, with significant impacts on transportation and, 
consequently, the national economy. A deliberate attack by a criminal group was hypothesized, characterized 
by successive actions that corresponded to corresponding reactions from existing security systems or the 
intervention of the police, with the aim of preventing sabotage of the equipment or, at least, arresting the criminal 
group. The case study, although linear and not complex in its outline, is extremely realistic and sufficiently 
comprehensive to maintain its generality. The event tree technique allowed us to identify the evolutionary path 
of the hypothesized dangerous condition, thus obtaining a clear attack sequence that, as previously mentioned, 
could be accompanied by responses aimed at reducing the damage. Unfortunately, however, the method does 
not allow for exact solutions in terms of the probability of occurrence of each scenario, so a computational 
analysis using the Monte Carlo method was used. The application of the numerical method allowed for tangible 
results, enabling the definition of the most likely scenario and the prioritization of risks. This allows for the 
development of strategies aimed at preventing such actions, using resources (financing, labor, time, etc.) that 
can improve both passive (alarm systems, gates, etc.) and active (police action) security systems. 

An important aspect that this article aims to highlight is that vulnerability analysis is essentially independent of 
the causes of the attack (terrorist attack, demonstration, industrial sabotage, etc.). What matters is the attack's 
evolution and its modalities, in order to understand how to intervene to limit the damage or even prevent the 
attack. The purposes of this sabotage can be extremely varied, from a terrorist attack to a demonstration, but 
this is completely beyond the scope of this analysis. For this reason, the article will not address the hypothetical 
causes that could drive this criminal group to carry out such an attack. 

Declaration of competing interest 

The authors declare that they have no known financial or non-financial competing interests in any material 
discussed in this paper. 

Funding information 

No funding was received from any financial organization to conduct this research. 

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