hi,
There's a couple of ways:-
1) open the "developer" tab in the Trainz launcher, the select "Show profile". This displays the FPS 9along with lots of other info).
advantage - it's built in, disadvantage is you need to have it open in a window over trainz - so everything is running "windowed" mode.
2) If you have nVidia GPU and you can run "GeForce Experience", then there's an overlay you can have (in the settings menu) that will display the FPS in a corner.
3) Download "Fraps" - the free version works for this - again, it will display the current fps in a corner.
Colin