  Engineering projects for railway infrastructure typically involve many
subsystems which need consistent views of the planned and built infrastructure
and its underlying topology. Consistency is typically ensured by exchanging and
verifying data between tools using XML-based data formats and UML-based
object-oriented models. A tighter alignment of these data representations via a
common topology model could decrease the development effort of railway
infrastructure engineering tools. A common semantic model is also a
prerequisite for the successful adoption of railway knowledge graphs. Based on
the RailTopoModel standard, we developed the Rail Topology Ontology as a model
to represent core features of railway infrastructures in a standard-compliant
manner. This paper describes the ontology and its development method, and
discusses its suitability for integrating data of railway engineering systems
and other sources in a knowledge graph.
  With the Rail Topology Ontology, software engineers and knowledge scientists
have a standard-based ontology for representing railway topologies to integrate
disconnected data sources. We use the Rail Topology Ontology for our rail
knowledge graph and plan to extend it by rail infrastructure ontologies derived
from existing data exchange standards, since many such standards use the same
base model as the presented ontology, viz., RailTopoModel.
