nothing. you can't die from inhaling too much smoke in a train tunnel because there aren't any train tunnels long enough to build up enough smoke to kill you. some will argue otherwise, but really, you have nothing to worry about; unless of course you stop in a really long tunnel and the engine keeps running (very unlikely) or you go through the tunnel between vancouver bc and points east, but even then i've known several people that have slept through it and didn't even notice.