I don't think I get your point. Presumably ventilation is easier to engineer near the surface, but air quality is always going to be bad if oil/coal/coke/smokeless coal are burnt (or are you saying that these fuels weren't used, which would be incorrect).
The claim is that the air in the cut-and-cover lines would be no worse than around a surface coal-burning train. Which could still be pretty bad, though probably not uncomfortably hot.
