I don't think the tunnels are cold they have an awful lot of heat being pumped into them from the trains passing through and no where for it to go. If condensation was a problem how come a cold train doesn't suffer the problem? Even a cold wet from rain trains don't appear to. And I'm sure there will have been times past with really cold snowy trains entering the tunnel and not breaking down. Don't underestimate the abilty of powder snow to penetrate, it's almost "if air can get through so can powder snow".
As for the snow not being a problem have a look at the end of this story:
Agreed, the suits have morphed from accountants to box tickers.