Ok
Train chess

they must get off after one turn.
That's a tough limitation. Especially when you're in check but instead of getting out of check you must get another piece off a train. We could test if it is fun without any such limitation.

1) No
2) No limit. Also I just realized that pawns wouldn't be able to promote, so now, when a piece gets in the way of a train, it will just be pushed in front of the train and will make the train stop early.
3) Kings can't get on trains.
what are the movement rules of the Guard. what is the rate of train movement? will a train stop until a piece moves out of the way? while I assume knights can pass over a train can any other pieces? also consider trains on 5th & 8th rank. even alternating between any and all of theses ranks.
Good Luck.
This is played on a 12x12 board, with two trains on the 6th and 7th ranks. These trains are 3x1 and go from one side of the board to the other every turn, they are neutral. Pieces can go on these train (room for 3) and ride them but cannot capture when on them, they must get off after one turn. If a piece gets in the way of a train, it will die and the train will proceed to it's destination (safety first!!) . The trains start at (abc,6) and (ijk,7). The pieces start at the same positions as standard chess except that there are two guards in between the rooks and knights and there's more pawns. Also, black's king and queen are switched.
Other than that the rules are the same as regular chess.