Another longshot, if you have Nvidia Experience running, remove it, that caused problems here, also the Win10 Game bar doesn't like Trainz much.
Got any other games you can try? may narrow it down to if it is a Trainz problem or driver / GPU one. If no suitable games perhaps the free trial of Passmarks performance test? It carries on working after 30 days with some of the more advanced stuff disabled that's aimed mostly at the benchmarking obsessed. https://www.passmark.com/products/performancetest/
Another thought the 3900s need two separate leads from the PSU, not one with multiple connectors, the builder may have slipped up there.
Maybe run GPUz at the same time and see what readings it's showing, also performance monitor and see if anything else is running, in particular on the TRS19 drive.
Got any other games you can try? may narrow it down to if it is a Trainz problem or driver / GPU one. If no suitable games perhaps the free trial of Passmarks performance test? It carries on working after 30 days with some of the more advanced stuff disabled that's aimed mostly at the benchmarking obsessed. https://www.passmark.com/products/performancetest/
Another thought the 3900s need two separate leads from the PSU, not one with multiple connectors, the builder may have slipped up there.
Maybe run GPUz at the same time and see what readings it's showing, also performance monitor and see if anything else is running, in particular on the TRS19 drive.