18xx.games is a website where you can play async or real-time 18xx games! Right now only 1889 is implemented but I'm planning on doing many more in the future.
You can play locally with hot seat mode without an account. If you want to play multiplayer, you'll need to create an account.
If you look at other people's games, you can make moves to play around but it won't affect them and changes won't be saved. You can clone games in the tools tab and then play around locally.
In multiplayer games, you'll also be able to make moves for other players, this is so people can say 'pass me this SR' and you don't need to wait. To use this feature in a game, enable "Master Mode" in the Tools tab. Please use it politely!