Montreal->Toronto corridor has complete coverage (unless you take the route through Windsor). 4G LTE the entire way.

In the SF Bay Area, Caltrain is aboveground and is more than quiet enough to call into meetings. Think NYC's MetroNorth, there's no reason you wouldn't be able to make a call until you get into the tunnels near Manhattan. On the other hand, BART is underground with signal repeaters just about all the way through - but it's very noisy so you could call in and listen but probably not contribute.

If your transit system is underground without repeaters though, I agree that you're out of luck. I can't imagine trying to take a call on the T.

Here in Taiwan, we also offer free Wi-Fi access across the High Speed Rails system (~180mph, ~200 miles of coverage), reliable enough even in tunnels, as of this year.