[WalterBright]

> Why would it be?

Building a new network would not be constrained by existing implementations. For example, you can't just drive the trains faster on existing systems. Everything would have to be redesigned/upgraded to do it - the motors, tracks, track bed, brakes, suspension, safety equipment, schedules, signals, everything.

[masklinn]

> Building a new network would not be constrained by existing implementations.

That doesn't answer the question. Why would the transport be "correspondingly faster" in a smaller tunnel than a bigger one, given both are new digs?

[toss1]

Because capacity needs are not infinite.

If the units/hour capacity can be met with a smaller tunnel running fast little pods on a single track, then boring out a larger tunnel that could fit perhaps three tracks would be redundant.

And that assumes that the pods could run on adjacent open tracks like railcars, whereas the intended design may require a smaller tunnel bore that is close to the size of each pod.

[spiralx]

Air resistance is greater in smaller tunnels though, and aerodynamic drag is the most significant factor on speed when considering underground high-speed trains. This is one of the reasons for building tunnels significantly larger than the size of the train.

[intended]

That’s assuming the current system speeds are being met - as I recall the New York subways are run at lower and lower speeds than capacity to avoid other problems and accidents.