I think I will start it now actually, as I have some time... so I best get on with mapping.
By the way, the rules as I see it are:
*2 carriages per contributor (they will be consecutive) made from the template I will make shortly
*You can use your own custom textures and monsters if you want, but try to avoid going overkill
*TIDs and so on are as you like, I'll make sure there's no collisions at the end - again, try to avoid going overkill
*jallamann, if he joins, can make the locomotive as one of his carriages - this must contain the end, as this is where we'll be fighting to
*I'll handle the background and the like, based on NJTRAIN - suggestions for the themes are welcome
*No new weapons
*Only one music file (I'm dead set on the conga by the way, if anybody wants to make it - else I'll go and find it

)
*TITLEPIC and end graphics can be made by a volunteer
*I'll handle the text file and end of level cutscene when we're all done
*Volunteers must submit their two carriages to me (made from the template obviously

) in a wad file which shares their name, to avoid confusion
*With the WAD, volunteers must also submit a text file listing their name they'd like to be credited under, editors used, any credits necessary to people who aren't them, me or Enjay (as we'll be crediting him massively

), e-mail address if you'd like yours listed and a name for your two carriages (for the cutscene at the end)
*Gameplay must all be on the carriages, so no mapping random areas that you get teleported to or anything

*I'll put it all together in roughly difficulty order when we're all done
*Try to avoid giving too much ammo or whatever, as you'll unbalance later parts of the train
Do these rules sound ok? Do they cover everything?
Obviously, the idea is that we all work from the same carriage template (don't worry, it'll be as bare as I can make it) and I'll put it all together at the end. Your part in it is however long it takes you to make two carriages, with gameplay and whatever. Theming for the carriages would be as you like obviously.