Railway systems are complex systems whose design and construction require many cooperating actors. Multiple models have been produced to reduce semantic heterogeneity and enable cooperation across many companies and persons. However, each model created has its own point of view regarding the railway domain and uses its own vocabulary. Therefore, it is necessary to investigate ways to align those models and to ease cooperation between actors. This paper first identifies the main ontologies proposed in the last two decades. Then, we study and compare those models and analyse the difficulties when aligning them. The results regarding our attempts to align identified railway ontologies are thus discussed.