This style was commissioned starting in the 70's, when population density was lower, there were fewer trains, and average travel time was higher due to suburban sprawl and most workers employed in the inner city; many spent 1 hour or more on the train each way.
There's a new metro project that looks more global.
Subway trains are designed to get people on and off the train as quickly as possible, with many doors and often platforms designed for quick transfers. Additionally, subways are designed for short rides, often with high stand/sit ratios.
Double decker trains are designed for long distance trips and to fit as many chairs on the train for a given train length, at the cost of number of doors and time loading/unloading passengers.
Yeah researchers don't always agree on things. Definitely a question up his alley though, since I think he's done a video with a similar theme.
Actually, I just watched the new RMtransit video on YouTube, and he shows that the RER A in Paris runs double Deckers in tunnels through the center. It's more of a commuter train, but it's very close to a subway, and the first I've seen of urban commuter trains like that with double decker rolling stock.
The main downside of double-decker train cars is the time it takes passengers to to board them. And, since this is one of the main factors limiting metro frequencies and thus capacity, they're not that suitable for subways. To maximize metro capacity, you want long trains with many doors and very high frequency.
Double-decker cars are much more suitable for lower-frequency service (S-Bahn, regional, long-distance,...) where they're also commonly used.
Of course, you could still use double-decker cars in a metro (and maybe some places do), it's just suboptimal.
It sounds like a weird idea at first, but maybe it could actually work. Kind-of like running two trains on top of each other instead of after each other. I guess the downside would be the need for bespoke rolling stock and larger platforms. I think, it would generally be preferable to double the frequency or run longer trains. But it could be interesting if you've already exhausted those.
The trains I take semi-regularly have a few stops with tiny station platforms, regardless of the length of the train. They will announce something like ‘if you are in the first few cars start walking back’. You better pay attention too or you’ll have to uber from the next station back to where you should’ve gotten off.
There's a balance to be made between the flow of people and the seating capacity of trains.
Single level with many doors will load/unload quickly, however there's barely any seating.
Two level maximizes seating at the expense of dwell times.
Nobody made a two level train with a focus on standing yet, so we don't have a real world example. If it's even possible because you need more headroom than usually available on double deckers
That said there are metro-like systems with double deckers. Paris and Sydney have already been mentioned. True, they are usually classified as suburban systems, but are very much used for city trips as well.
I think one of the bigger things keeping metro rolling stock from using double decker units is the need to go underground. Increased height means larger tunnels or deeper cuts and that can get cost prohibitive. This is especially the case when simply lengthening the tunneling needed for longer station boxes/platforms to support higher capacity single deck trains is less costly.
As subways are usually intended for traveling short distances, the passengers have to get in and out fast. Thus, subways usually have doors in shorter distance from each other than e.g. in train trolleys, that are used on lines where the stations are in larger distance from each other than subway stations usually are. The trolleys of double decker trains have stairs close to the doors, thus the trolleys for subways would need to have equivalently more stairs. Subsequently, the space gained for passengers to sit or stand would be much less than e.g. for double decker trains.
Most subway lines were dug after the city they go under was built, and, for example, there's a whole lot of London on top of the London Underground. Very difficult to dig upwards, very expensive to dig downwards. In the above ground sections you'd have to rebuild all the road bridges.
Much easier and cheaper to run the most efficient service possible with a high throughput of trains.
High-frequency systems with shorter trains are often more effective at moving passengers quickly during peak hours, because the trains come more often, reducing platform overcrowding and wait times. For example, the Tokyo Metro system, despite having relatively shorter trains compared to New York, serves a much higher passenger volume because of its incredibly high frequency and reliability.
No one has mentioned the idea of having double-decked stations to make it so you don't have to worry about moving to the right floor in advance of your stop.
I would posit this as a given if we bothered to make taller tunnels for taller train-cars, but based on other comments here, I'm not sure this idea actually makes the concept of double-decker subways any more sensible or useful.
It looks like those metros that have a use for the idea have made it work in their own way and places that haven't tend to have significant reasons of their own for not going this route. Enlarging existing tunnels vertically seems to be a non-starter in most places, for instance.
Here in Boston a lot of the commuter rail cars are double decker. I guess it’s an upgrade to carry more people rooms but a better upgrade would be running more trains. That costs money though
Like other people said, I don’t know about subway. You want to be quick on quick off. I don’t usually even sit (even when there’s room )