ProRail is the railway management company of the Netherlands. It is responsible for the construction, maintenance, management, and safety of the Dutch railway system. ProRail has as a goal an annual five per cent increase in the number of transported passengers up to 2020. This goal cannot be reached with the current way of transporting passengers. There is no room to expand the infrastructure, so the manner in which it is used must change. This means that during the decision making, the impact on individuals connected to the system must be taken into account. ProRail makes these decisions in collaboration with other actors. This means that ProRail is involved in a process of multi-actor decision making. This is an uncertain process. The uncertainty stems from three main causes: substantive uncertainty (ambiguity and lack of information on a project), strategic uncertainty (unpredictable behaviour of other actors), and institutional uncertainty (unknown decision making procedure). Because the Dutch railway network is a socio-technical system, the substantive uncertainty stems from social elements, technical elements, and exogenous elements. As no clear definition has been found in literature, this research proposes a definition for these elements based on the way in which an element can respond. A degree of freedom (DOF) is defined as the possibility to respond in an allowed way that is not included in the task description of the element. Technical elements are designed with zero DOF and social elements with more than zero DOF. Exogenous elements are elements that are not designed in the system but do influence it. In this research the value of the HAZOP methodology for decision makers at ProRail involved in multi-actor decision making is assessed based on literature and a trial application. To make this assessment the causes of uncertainty in decision making in projects initiated by ProRail are investigated. Second, the methods and practices currently used by ProRail to reduce uncertainty and make decisions are evaluated. Third, the potential of the HAZOP methodology to reduce substantive uncertainty was assessed. Last, it was investigated if the HAZOP methodology could improve the decision making process. ProRails tasks can be subdivided into two core tasks: (1) the daily operation of the network including controlling and correcting the flow of trains; (2) the execution of projects. The methods and practices currently used by ProRail to reduce uncertainty in decision making in projects do not sufficiently reduce the level of ambiguity. This results in a level of uncertainty too high to make good decisions. This study proposes the HAZOP methodology as an alternative decision making tool for decision making on projects and explores its value for decision makers at ProRail involved in multi-actor decision making. The HAZOP analysis and a suitable two-phase method of interaction are proposed and discussed based on a literature review. The first phase of this method is an individual preparation. This phase reduces the probability that important elements of the system and variables are overlooked. The process of combining the individual definitions into a joint image of the problem in the preparation phase allows a reduction of the ambiguity of the problem. The second phase is a group-based discussion. This discussion allows tacit knowledge to be made explicit for the other members of the group, and allows the joint creation of knowledge. Both of these processes reduce the lack of information. It is thus theoretically feasible to reduce uncertainty using the HAZOP methodology. A trial session was organised to apply the methodology and validate the theoretical results. The results from the application support the results from the literature review. The creation of a joint image was observed as the combination of different definitions into a single shared definition. Ambiguity was reduced, which was observed by an increased ease of communication, especially on more technical subjects. Social subjects remained a difficulty, but this could be caused by a lack of expertise of the participants. During the discussion one of the participants shared previous experience and made his knowledge explicit. This allowed the other participants to change their definition, a process that was not seen in literature. The analysis could not be performed due to time constraints, therefore a reduction in the lack of information and the joint creation of knowledge were not observed. However, the new information becoming available by making knowledge explicit makes it plausible that the lack of information can be reduced by the HAZOP methodology. To assess the value of the HAZOP methodology the chance that the advice will be accepted was investigated, as well as whether the methodology has a positive effect on the decision making process. The acceptance of the advice could not be predicted because it was not possible to predict a priori the trust decision makers will have in the advice. The value was assessed based on the feasibility to include joint fact-finding, redundancy, and parallel linking. In the current form, the methodology allows for joint fact-finding and to a limited extent redundancy, parallel linking seems infeasible due to the required time of the analysis. The conclusion of this study is that the value of the HAZOP methodology in the current form is too low to implement it. However, it is a promising tool for the reduction of uncertainty in the decision making process. To increase the value of the methodology, the time required for the analysis should be decreased. Limiting the required time seems to be feasible by limiting the discussion time per deviation. This is one of the recommendations for future research made at the end of this research. The results of this research present key insights for the decision makers at ProRail. The first is that the current methods and practices of ProRail of reducing uncertainty are insufficient because they do not reduce ambiguity. Reducing ambiguity requires the collaboration of different departments or with different external parties. Second, two dilemmas were identified that should be recognised and considered prior to initialising research. These dilemmas are (1) the choice between better suitable and more familiar participants and (2) the choice between a faster or a more thorough analysis of the alternatives. These dilemmas are not unique for the HAZOP and thus do not speak in favour or against it. Last, the definition of social elements based on the task description rather than the fact that an individual performs the task allows for a closer look at the task descriptions of the elements.