Strange things... I'm currently doing the final testing of my absolutely gigantic and ultra-detailed Great China 4 (yes, yet another) scenario for TrainMaster with 380 (!) engines currently running between 200 cities, all graphical details maxed, at windowed 1440 resolution. I get 30..80 (occasionally 100+) FPS. And no, although pretty fast, this isn't an ultra-expensive supercomputer. There must be some simple reason why similar-looking computer setups behave so differently with RRT3.
One possible thing... Since this is an old game, it probably relies on some pretty old supporting technology and software, such as old versions of DirectX. Could it be that RRT3 takes some of this software for granted although modern Windows 7 doesn't include or install it anymore? If that's so, then I must have installed some of that stuff accidentally. I also still suspect that Microsoft's optional (!) compatibility updates for W7 may well have something to do with this. If they help other "old" software to run on W7, why not RRT3, after all?
Have you guys also tried running in both windowed and full-screen mode?
AMD Phenom X6 1090T @3.9GHz, 16GB DDR3-1600 RAM, Asus Crosshair Formula IV mb, Radeon HD7870, Samsung 850EVO SSD, M-Audio AP192, Windows 10-64, Railroad Tycoon 3 1.06. & TM, Train Simulator 2016, MSTS + many add-ons, Trainz!