I assume you've tried quitting and restarting? If so, this might be a 'fun' save file to send to Notch - he might be able to fix it and/or might be able to solve the bug for the future.
Well, I closed it with the taskmanager, because I didn't think it was a good idea to save it like that. Hopefully nothing is too fucked up. I'm a little worried now, I don't want to throw away this save after the assload of work I put into this railroad.
I've never tried saving while inside a mine cart... does the game actually
remember that? I'd have assumed that you'd re-enter the world standing in place, probably with the mine cart shooting off into the distance from the impromptu collision with a player. Sadly, none of my saves have mine carts or enough free iron to test this.
If you C-A-Ded it, the state of the save really depends. If the game wasn't actively saving a chunk at the time, then you'll probably be fine - you might lose some recent changes. If it
was saving, it'll probably still be playable, but likely with corruption - either half-saved chunks, maybe some repositioned chunks. I don't know if its possible to corrupt MC maps to a non-playable state, but if so, you could manually delete the damaged chunks (or ask someone to do it... heck, there might even be a program for it, but I haven't followed the MC forums enough to know).
Of course, manual recovery is only an issue if the map refuses to load - as long as you can get in-game, it'll probably be fine.