It's also that they don't cool the tunnels like they really really ought to (just feed frozen water into the tunnels and drag the meltwater back out if it won't

It's also that they don't cool the tunnels like they really really ought to (just feed frozen water into the tunnels and drag the meltwater back out if it won't drain by itself, say by fitting some cars with slush sprayers like how manure is sprayed from the back of a tanker on agricultural fields, or dropping ice cubes spread-about). Use ice-based AC for the passengers, not heat pumps dumping into the tunnels. Change the breaking resistors to not be on the train, but the rectifier stations, and make them vent outside. Or change them to dump heat into what's a big insulated kettle that gets filled with ice or at least disposes the resulting hot water outside, not hot air into the tunnels.