via this thread on the Steam forum
There seems to be an issue where after playing the game for some time, it just locks up and then crashes. After some thorough research, it appears to be an issue where the game does not have the required setup to be allocated certain portions of your ram. In order to fix this, we must set it up so that it can get to this ram.
Disclaimer: Modified .exe's are being used with Steam, which may be a hazard to your account. However, I asked around on the forum and many people have done it and said it's fine. In addition, I am using a modified .exe myself, so I sincerely believe it will be fine.
This .exe has the fix already applied to it. Replace your railroads.exe in "C:\Program Files\Steam\steamapps\common\sid meier's railroads" with the downloaded .exe