It is an issue, but an ICE burns far more fuel than a human. (food is fuel that the body burns!) That means if your have ICE engines in a tunnel you need a lot more air to get rid of the CO2 (ideally no other pollutants - though I wouldn’t bet on this) and get enough O2 there for everything to burn. If we only run electric trains we still need some air for the people to breath, but much less as they are breathing much less