> Also: this thing being wider gives it about three times the floor area relative to a train, so it would require about thrice the number of doors to get passengers in and out efficiently. How do you do that while keeping it structurally safe? I guess the answer is that that spacious room inside would see a few walls added.

That thing has already twice more doors than a train since the doors of both sides can be used, so it partially solves that problem already.