It works ... but ...
The main problem is triangle count. In Railworks, for secondary objects like a tree, we aim for a triangle count of a few hundred to a few thousand triangles at most. We want to reserve our triangle budget for the heroes in the scene - the trains.
Sapling creates very high detail trees, typically a hundred thousand to a million triangles. Its algorithm creates four or more triangles per leaf.
Its possible in sapling to thin out the number of leaves, and reduce detail settings to get into the tens of thousands range and then the tree might load into Railworks for a demo scene. But still, it doesn't make a good model for use on a real route because its hogging too much of the GPU.
I had a bit of success with a method where I replaced the leaf texture with a full branch texture. And then turned the leaf density way down. That may be worth experimenting with.
Wayne