There are two things that control the ruling headway on a subway system. The first is platform dwell time. While a train is stopped at a station, you cannot let another train move into its position as if it was going to start moving on schedule, because it's actually quite likely that it won't. Given that platform dwell time is about 30s, and taking into account time needed to decelerate and accelerate to/from a stopped position, this limits minimum headways to about 1m.
Similarly, switch fouling time is another constraint: you don't want to move a switch while a train is allowed to path over it, in case the switch fails to switch. From what I've heard of existing urban systems, this leads to a minimum headway of ~90s, although I don't know how much of that is signal-induced padding.
The next thing to point out is that these are theoretical maximum density; the practical maximum operational usage is generally far less. Most subway systems have the physical capability of operating ~45TPH on a subway line, yet you'll notice that extremely few do. Moscow Metro has managed 40TPH on an unbranched line; branched lines struggle to get to 30TPH, and heavily reverse-branched systems like the DC Metro or NYC Subway struggle to make even 20TPH. Introduce branches into the mix, and you need trains to make it onto the mainline in slots, and there's going to be variance in arrival time because the system is used by humans; reverse branches makes the problem worse because these slots need to line up well on multiple lines at the same time.
> Trains would be able to hitch and unhitch whilst moving 100 mph, allowing different loads to be sent different directions and different sets of passengers to stop at different stations.
No. There's a reason railroads have banned the practice of unhitching at speed (it's incredibly dangerous), and hitching is even worse. And if you're talking about EMU passenger train sets, most of them are designed to not be hitched or unhitched particularly frequently--these aren't your standard automatic coupler system (which doesn't couple brake lines or electrical lines or other things automatically anyways, FWIW).
> and heavily reverse-branched systems like the DC Metro or NYC Subway struggle to make even 20TPH.
I find this really interesting that is seems to be the limit for "big-boy" complicated subway systems, while there are many very complicated smaller systems that achieve much more. For example, a lot of the german Stadtbahn systems run somewhat long trains (up to 80m), run using conventional signalling systems underground (so not on-sight, which would allow for a much higher throughput). Naively I'd assume the tram-style segments and frequent at-grade crossings would make this much worse, but apparently not.
For example, Stuttgart (https://download.vvs.de/Stadtbahn_Liniennetz.pdf, https://gleisplanweb.eu/show.php?Map=Stuttgart&Index=1&Heigh...) currently runs 27 tph between Stadtbibliothek and Olgaeck, with plans to run 30 tph without any upgrades, and 30 tph between Staatsgalerie and Stöckach (additionally even running the U11 for events) without any significant issues and quite a complex network with many flat junctions outside the underground sections. The DC Metro has much less complicated branching/reverse-branching patterns.
No, they don't. There are a handful of weird french systems (and even less outside of France), but there are more ruber-tire metros than trams, I'm sure. They are rare and have been getting rarer.
80m is in the lower half for metros, but long in the context of systems having on-street portions (exceeding the german legal limit of 75m with a special exception). It's also on the long end for small metro systems.
Similarly, switch fouling time is another constraint: you don't want to move a switch while a train is allowed to path over it, in case the switch fails to switch. From what I've heard of existing urban systems, this leads to a minimum headway of ~90s, although I don't know how much of that is signal-induced padding.
The next thing to point out is that these are theoretical maximum density; the practical maximum operational usage is generally far less. Most subway systems have the physical capability of operating ~45TPH on a subway line, yet you'll notice that extremely few do. Moscow Metro has managed 40TPH on an unbranched line; branched lines struggle to get to 30TPH, and heavily reverse-branched systems like the DC Metro or NYC Subway struggle to make even 20TPH. Introduce branches into the mix, and you need trains to make it onto the mainline in slots, and there's going to be variance in arrival time because the system is used by humans; reverse branches makes the problem worse because these slots need to line up well on multiple lines at the same time.
> Trains would be able to hitch and unhitch whilst moving 100 mph, allowing different loads to be sent different directions and different sets of passengers to stop at different stations.
No. There's a reason railroads have banned the practice of unhitching at speed (it's incredibly dangerous), and hitching is even worse. And if you're talking about EMU passenger train sets, most of them are designed to not be hitched or unhitched particularly frequently--these aren't your standard automatic coupler system (which doesn't couple brake lines or electrical lines or other things automatically anyways, FWIW).