An off-the-cuff list of online resources & methods for playing the 18xx:
- CyberBoard (many fan-made kits, a file-sharing system to move the save-files between players, and with a spreadsheet for the numerics)
- ps18xx (file-copying with a spreadsheet – link is to a version of Stephe Thomas’ rather improved version of Matthias Klose’ original)
- Rails (file-copying the save-file)
- rr18xx (requires a login code that can be found in the 18xx mailing list archives)
- Stephe Thomas’ email-based “Mais n’est-ce pas la gare”.
- VASSAL (many fan-made kits and with file-copying etc)
I use ps18xx heavily and don’t use any of the others.
I’ve also setup games under a webcam and played via a Google Hangout, which worked fairly well.