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.
Pointing to this whenever the conservatives in Sweden say we need to raise prices and hire more ticket cops in order to increase ridership