I would stick to the OSM design as close as possible. It works pretty well accross most zoom levels.
I use #787878 for the base colour. Width 1mm. For the dashed line on top I use white as colour. Width 0.6mm. Cap style Square. Custom Dash 1.5mm (Space 3).
I'm not sure these sources have exactly what you are looking for, but they may be good sources:
Ordnance Survey Open Data website.
TFL Open Data API includes "Routes and lines (topology and geographical)"
GTFS Data Exchange has information on a large number of rail agencies.
A search on data.gov.uk for rail returns several datasets that you might find useful.
For example Network Rail - railway network Inspire data
There's a WFS and a WMS for the rail network and for stations
Here's a preview
I am just going to expand on my answer above.
You can use the list to reference what exists and how it is structured What Rail Station Code Should I Enter? In there you will see examples of "GBQQS"
London St Pancras International (GBQQS) GB GBQQS
as well as snippets of
DÃœSSELDORF FLUGHAFEN (DUS) (DE@DF) DE DE@DF
give that it can be queried I can ...