My 2 cents...
The lock, from gate to gate, is 300 feet. That's a football field, goal line to goal line.
Never heard the "car" term before, but I'm thinking it might refer to three barges side by side.
So the lock can handle 3 cars at a time, with a little room at the tug end and enough room at the front of the raft to swing the lock gate closed. which is a work of modern engineering since the gate (2 halves) get expanded as they swing shut by about a third its closed length. Took me a long time and a lot of watching to figure that one out.
So....a twelve barge raft takes two movements (three car lock-thru and one car and tug lock-thru).
The cam hasn't stayed with very many complete barge and raft moves, so I haven't seen many and no 15 barge rafts, but I know the Corps of Engineers is supposed to lengthen all the locks on the Upper Mississippi to 600 feet lock gate to lock gate to accommodate a speedier passage for 15 barge rafts. And that was supposed to be competed by 2023.
And those big nearly 600 ft. cruise ships that Viking has been building might have some influence too!
Maybe my two cents got a little wordy, but I've thought about it a lot and hope there are "experts" out there with more conclusive information.
