The Unified Modeling Language (UML) is widely used as a high level object oriented specification language. In this paper we present a novel approach in which reverse engineering is performed using UML as the modelling language used to achieve a representation of the implemented system. The target is the core logic of a complex critical railway control system, which was written in an application specific legacy language. UML perfectly suited to represent the nature of the core logic, made up by concurrent and interacting processes, using a bottom-up approach and proper modeling rules. Each process, in fact, was strictly related to the management of a physically (resp. logically) well distinguished railway device (resp. functionality). The obtained model deeply facilitated the static analysis of the logic code, allowing for at a glance verification of correctness and compliance with higher-level specifications, and opened the way to refactoring and other formal analyses.