Are you sure that you only have the bare minimum amount of content installed? Ideally you should have just the consists, the locos/whatever needed, and the textures etc within the TRAINS folder.
And be sure that you only have the route that you wish to run present in the ROUTES folder. That's what enabled it to work for me. IE bare minimal content to load. It's a game designed for ME/XP, not newer systems obviously.
TS2020 still works without any real problems, as does Train Sim Modeler. Although with TS20XX, you do see the not responding indication when saving sometimes, but I've never seen it closed by Windows just because it was taking to long to write to the content folders.
Ideally, there should be a batch file or something which moves only the files that are needed for the activity into the MSTS folder, with the rest in another folder that MSTS doesn't require.
Edit: If you have a separate installation on a USB drive or something with the train.exe files, try dragging it into somewhere that isn't within the program files folder (IE in a folder on the desktop), and see if it works. I am not responsible for any security issues you may face as a result of doing this. It is merely a suggestion. All I know is that miraculously, it somehow works (or did at one point) on Win 10.
I have all the default routes, trains and consists installed! I never read anything regarding a minimum amount of content to get MSTS running on Windows 10?
The only Aussie content I have installed now in the MSTS folders is Marek's excellent NR loco's and IP/Ghan carriages for use in Open Rails!
As for TS2019, I don't have any issues running this game on Windows 10
I can however, run previous pc games on Windows 10 from the Windows XP era such as Alien VS Predator 2, Sim City 4, Trainz 04, 06 and Ultimate and Flight Simulator 2002! But not MSTS
I also have no issues running Train Sim Modeler on Windows 10
I have never ran MSTS from a USB drive, I also don't have one at this stage! So I can't test that out....
As for any security prompts, I don't get any! I have fully disabled that UAC crap on Windows 10 same with Windows 7, it causes a lot of issues with MSTS itself as well as other pc games I have installed especially when auto-saves for Train Sim Modeler, Blender, 3ds Max, Maya, Google Sketchup and other modelling programs needs the auto-save function to work correctly! But the main issue is that I noticed that when the UAC is turned on TSM will not auto-save at all on either Windows 7 or Windows 10! Simple fix
I will try running MSTS with just one default route installed "I will just move the other safely to desktop" and get back to you!