If the routing is done with an internal database (rather than some roads-only API) it might be straightforward to use the railway lines from Open Street Map [1]

If the routing is done with an internal database (rather than some roads-only API) it might be straightforward to use the railway lines from Open Street Map [1]. Or even the public transport routes recorded in OSM [2]. [1] https://openrailwaymap.org/ https://openrailwaymap.org/ [2] https://www.openstreetmap.org/#layers=O https://www.openstreetmap.org/#layers=O