[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.