But it’s not an odometer. It’s a sensor on the track that counts axles. The system knows the precise location of the sensor, and how far axles are spaced.
That's interesting. Do you know if they are used to keep track of the trains' positions with axle-spacing precision everywhere, or only at stations and track-section boundaries? (my somewhat cursory search suggested probably the latter.)
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
