He has specifically said they are making smaller tunnels to dig faster and will use transportation tech that fits in smaller tunnels to take advantage of this efficiency. I think many industries could benefit from this kind of thinking. "Because that's how we've always done it" is a great way to stagnate
I think the grandparent is nitpicking the "faster" claim here. To make your comment more specifically address that, Musk isn't claiming to have technology that can bore 10x more volume than competitors. He's reframing the market needs from cubic volume bored to distance bored.
But that really isn't the metric you actually care about. The only things that matter is distance tunneled per time and that the resulting tunnel can handle your transport needs. Musks believe is that the transport needs can be handled by smaller diameter tunnels than everyone else is boring. At least if they also built the "trains" and casually looking at subway networks that very well might be true.
Why would it be? If you can run fast in a small tunnel you can run just as fast in a big one. The size of a tunnel is a factor of what you want to do with it and what you want to run through it. TBM manufacturers can scale down to 3ft if that's what you need.
Fun fact: a subway trunk line is more frequently physically occupied by a train than a highway lane is by a car.
The factors that limit train frequency are station dwell times and switching time. A subway line can generally hit 26TPH, and the top speed of most subways is usually about 70mph, with average speeds generally being in the realm of 30mph. Making trains faster actually reduces capacity; a HSR that goes 220mph is considered to have a capacity of around 4-6TPH. You can also improve throughput by cutting out all branching; Moscow gets about 40TPH as a result, which is about the feasible limit of rail systems.
I’ve been to Moscow twice in the early 2000s. The metro system there is still my mental benchmark for a metro system. The scale of everything is impressive. The number of people moved as well as the grand scale art. Say what you will about the soviet system, but you have to give them style points for one hell of a metro.
30 MPH which is low = 5280 * 30 = 158,400 feet per hour / 26 TPH = 6092 feet per train. Actual subway trains are 600 feet or less long ~= 10% utilization. At 30MPH cars don't keep 126 feet between each other. Bump that to 60MPH and the trains are at 5% or less utilization and again cars don't keep 266 feet between each other even if they should. And again this is very long 600 feet subway trains most are significantly shorter than that.
Further trains have a stopping distance @ 62MPH of 820 ft with (0.15 g) deceleration. High speed trains can get an extra 0.3 m/s2 deceleration which could also be added to normal subway trains but would be an emergency situation as they knock people over.
PS: At 60MPH the theoretical limit is over 220 trains per hour assuming all trains can stop before hitting the train in front of them. But you can only approach that with full automation and multiple lines for acceleration.
The Victoria Line in London runs at 36 trains per hour at peak, one every 100 seconds. Station dwell times are already a limiting factor at this point, and having separate load/unload platforms would provide only a small decrease in dwell time as you'd need to open the unload doors before the loading doors to prevent people just exiting the wrong side.
You can have multiple platforms and send trains to each not just separate load/unload platforms. This was very common with passenger trains as boarding times could extend for long periods. Even when it was a single line for loading and unloading they would normally have a bypass line to avoid the station.
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.
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.
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.
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.
If you can still get people through the smaller tunnel you have dug more quickly (because you're using smaller, low-profile vehicles on 'skates') then it's not snake-oil, it's smart.
Don't forget also being able to handle evacuations and emergency situations. A lot of tunnel design isn't just getting a train through, it's getting people out.