> Which Track? Trains don't randomly switch tracks except in yards,
Track spacing is often much less than the dilution of position, which is less than ideal. This means that the area that the GPS thinks the train is, is bigger than the track width. Trains in the UK are often in cuttings, urban canyons etc, which means that the error margin could be up to 100m without much effort.
> instrumenting each switch to understand when a train switches tracks seems like an easy problem.
Only if you ignore that those sensors are now life critical, so is the connection, and the software that joins it all up.
> Tunnels? First there aren't "that" many and there aren't many long enough that a single long train shouldn't count it as occupied in the interest of safety.
again see DoP. But safety distance is related to speed. the slower the trains the shorter the distance, which means you are more likley to bump into GPS DoP issues. Also tunnels normally have cuttings, which again limit the number of satellites visible.
> It also shouldn't be a high effort to add a satellite or wireless repeater with a known location at at least each end of a tunnel.
again see safety critical equipment, you'll need fail safes
> The accelerometer can even early report rough tracks or switches.
Whats the percentage drift per second of those sensors? How often is the calibration frequency? how much vibration can it handle before it becomes inaccurate? whats the mean time to failure?
As a point, the "new" thameslink trains refused to open its doors at the thameslink station, because the station was in a tunnel, and the GPS wasn't available. This meant that the train said it didn't know where it was, so no doors open.
As a general rule of thumb, life critical stuff needs to be simple, well tested and well characterised. Failure of a system might cause a death, subtle failure is even more dangerous. This means expense.
Track spacing is often much less than the dilution of position, which is less than ideal. This means that the area that the GPS thinks the train is, is bigger than the track width. Trains in the UK are often in cuttings, urban canyons etc, which means that the error margin could be up to 100m without much effort.
> instrumenting each switch to understand when a train switches tracks seems like an easy problem.
Only if you ignore that those sensors are now life critical, so is the connection, and the software that joins it all up.
> Tunnels? First there aren't "that" many and there aren't many long enough that a single long train shouldn't count it as occupied in the interest of safety.
again see DoP. But safety distance is related to speed. the slower the trains the shorter the distance, which means you are more likley to bump into GPS DoP issues. Also tunnels normally have cuttings, which again limit the number of satellites visible.
> It also shouldn't be a high effort to add a satellite or wireless repeater with a known location at at least each end of a tunnel.
again see safety critical equipment, you'll need fail safes
> The accelerometer can even early report rough tracks or switches.
Whats the percentage drift per second of those sensors? How often is the calibration frequency? how much vibration can it handle before it becomes inaccurate? whats the mean time to failure?
As a point, the "new" thameslink trains refused to open its doors at the thameslink station, because the station was in a tunnel, and the GPS wasn't available. This meant that the train said it didn't know where it was, so no doors open.
As a general rule of thumb, life critical stuff needs to be simple, well tested and well characterised. Failure of a system might cause a death, subtle failure is even more dangerous. This means expense.