To do this cheat you must you have a sceneic add on for MS Train Simulator. Click on any electric train route - then click on SRW train route - then electric train will be on a diseal route.
Go 200+ mph
Select "Simple Controls" in the options window, select the Innsbruck to St. Anton route, select the starting point to be St. Anton, and select the Flying Scotsman as the train. Start the simulation. Detach from your train (leave the tender) and press D until the regulator is at 100%. Wait, and your speed will exceed 200 mph.