I don't know for certain, but they'd have to have at least one set on both tracks in each ventilation section, to enforce ventilation rules (read the rest of th

I don't know for certain, but they'd have to have at least one set on both tracks in each ventilation section, to enforce ventilation rules (read the rest of the article about the tunnel's ventilation management). It also points out that axle counters can sense backward movements, should a stuck train require the ones following it to reverse out of the tunnel