Subjective statements like T:ANE runs better on NVidia cards (rather than AMD ones) are anecdotal rather than empirical. There is simply no evidence to actually support these assertions.
However, there is right now a significant gulf between the two big video card manufacturers' highest-end performance card offerings. (Pascal vs Polaris architectures).
To illustrate this, a quick glance at the Tom's Hardware Desktop GPU Performance Hierarchy Table will highlight the fact that there really are not any single-GPU performance equivalents for the GTX Titan X (Pascal), GTX1080 and possibly even the GTX 1070 cards from either Intel or AMD right now. (Upcoming Vega units may well provide better competition in 2017).
http://www.tomshardware.com/reviews/gpu-hierarchy,4388.html
On the other hand, in the mid-range, I suspect that the perceived performance and relative smoothness of T:ANE game-play in a double-blind test of an RX-480 vs a GTX-1060 would be practically indistinguishable for most Trainz users.
And older AMD architectures like the R9 Fury X still run T:ANE exceptionally well (albeit at a higher cost per frame-rate than the newer Polaris GPUs).
So go for what makes economic sense to you, after consulting the comparative reviews. Some AMD cards are considerably superior in terms of performance (and T:ANE gameplay experience) to older Nvidia cards and you can see the rough equivalents in the venerable Hierarchy Table.