A quick solution using AutoHotKey. Install AutoHotKey. Then create a new text file with the following:
Put this in a file, call it "pause.ahk" (if using Notepad, include the quotes or it will save it as pause.ahk.txt) or similar. Double click the file to load it. Now while in Railroad Tycoon, you can use the accent/tilde key instead of pause.
If you want to bind it to another key, just put another key before the :: in the script above. Almost any key works, for some of the more complex ones (ie, F-keys or the number pad) refer to this page.
When you want your key back, just quit the script by right-clicking on the green "H" icon in your system tray, then choose Exit.
It's also possible to make it only activate when Railroad Tycoon 2 is the active window, by adding this line to the top:
#IfWinActive ahk_class RT2_TSC
... assuming you've got the same version of RT2 that I do.