Which version of Trainz are you using? This is the TRS19 forum so I would guess that it is TRS19 but to my knowledge no-one has posted about a similar problem.
What track asset(s) are you using - kuids would help.
An image would also be a great help.
This sounds like the PBR texture problem. I think one resolution was to go to Trainz Settings Performance tab and change Shader Quality to standard. Maybe someone can confirm, or you can just try it.
