Train Sim World 2

It is hard to get a good stretch at a respectable speed and gradient on RSN. What I suggest (and this can be used for most other "x distance" achievements when the loco is specified) is to load up a high speed route like Munchen or the vanilla route with the ICE3 on it, then create a custom service going from one end to the other with the loco you want to work on. You might not need to switch to 'off the rails' but that may or may not prevent you from using your desired loco.

Start the custom service then set the throttle to max, at this point you can save the game when you get to 60 kph, that is its max speed. Then do whatever and set a timer until you reach the end or get SPAD'ed. When you reload the save you can now step away and only need to restart it when that set timer goes off. Still takes a while but is 99% hands free.