This guide is more or less equivalent to the OpenSpy generic patch[openspy.net].

Railroad Tycoon 3 multiplayer has been broken since the GameSpy servers were shut down in 2014. There is a patch / external software in another guide on here that claims to make it work, but it seems like those servers are now down.

Thankfully, OpenSpy[openspy.net] provides a replacement for the GameSpy servers with one-to-one compatibility. Setting it up requires hex editing the RT3.exe executable, which might be scary to some people, but should work without issues as long as you follow the steps exactly. I would recommend those playing with friends to edit the executable on your end and send it around instead of having everyone edit their own, just to prevent issues!

This should work with any version of the game, with any patches or mods installed, whether you got it from Steam, from GOG, or from the CD itself.

The steps are as follows:

1. Only if you're using the Steam version: the RT3 executable that ships with the Steam version seems to be encrypted or otherwise not directly editable. Before following the rest of the steps here, you'll need to replace RT3.exe with the original version. This guide describes how you can do this. Note that you only need to follow the first "Steam version: Replace RT3.exe" step of that guide for this to work - but installing the 1.06 patch is worth it, as it fixes some bugs and allows more custom maps to be played.
   
2. Download a hex editor. HxD[mh-nexus.de] is free and Just Works™️
   
3. Open up the game directory (right click on Railroad Tycoon 3 in Steam -> Manage -> Browse local files)
   
4. Copy RT3.exe and paste it in the same folder. This will make a copy named RT3 - Copy.exe. This is a backup in case you mess anything up.
   
5. Open up RT3.exe in HxD. It'll look like this:
   
6. Press Ctrl+F to open up the Find window and search for gamespy.com:
   
   
7. Press enter to search. The first appearance of "gamespy.com" will come up.
   
   
8. Type in "openspy.net" so that it exactly replaces "gamespy.com":
   
   
9. Repeat for the other appearances of "gamespy.com". There are three, by my count.
   
10. Save the file.
    
11. Open up Railroad Tycoon 3. Multiplayer should now work perfectly!