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.
worth asking RMtransit on Mastodon, though.
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.