I guess if you make it white / black in relation with background and other tracks, you should be fine.
Plus even a white line would be denoted in some ways in other non-digital places, like billboards, information boards etc. You can take a cue from that data. The brick and mortar presence should be very similar to how the app depicts it. This is essential for relativity (and branding if applicable)
Only FFF does not make a white, if you have a blue/red/white then a very light color easily differentiated from rest of the tracks and background should be good enough to denote white color. Same for the black color too.
The HTML color roaster has many light colors which coupled with borders and strokes should make the trick.
A more radical thought would have a separate deployment based on country, and replace the train assets with a circle of W, G, B etc., but scratch that, it is a very last resort. I do not much like it myself.