[jcranmer]

Or, for 10% more cost, you could build a regular subway and move the same amount of people in much less space. You even reduce the truly expensive part of subways (i.e., stations), since you only have to have a mezzanine that covers two tracks rather than 20 tracks.

[badwolf]

Think about it from a redundancy standpoint.

Single big trains get delayed due to any number of reasons - something on the tracks, broke-down train, etc...

With 10 smaller tunnels, they can just be rerouted.

[jcranmer]

Not really. If the other tunnels are essentially full up with other trains, then there's no space to squeeze in the trains from the blocked line. Furthermore, you'd have to have crossovers in place to enable that kind of routing, and those underground switching crossovers are not going to come cheap.

[Robotbeat]

Or just build longer pods. Or link them together like a train. Can still be narrow.