You hit it with your fourth shot. The usual situation is that the TBM is sitting idle a good portion of the time, because spoil can’t be removed from the tunnel fast enough. The problem usually isn’t getting it out of the tunnel, but rather getting it away from the tunnel head. Most TBM’s are working in areas where the transportation infrastructure is a problem, otherwise, what need would there be for the tunnel?
Secondary issues are things like providing power, and having enough room to work behind the TBM (doing things like installing casing and pumping out the water).
There’s also the problem of keeping the city running while you’re dismantling and reassembling the underground services in the machine’s path.
Actual performance of the machine is very seldom the limiting factor.
Oh, and I almost forgot to mention the issue of getting the thing to the work site in the first place. The existing infrastructure often won’t support rolling such a thing on the rails or through the city streets. Just getting the machine in and out can be a bigger job than digging the tunnel.
