[mungoman2]

> Naturally, trains already have track-based location systems, but they are usually based on a train being within a “moving block”, so their accuracy is down to metres rather than centimetres. If you want to monitor track conditions, the more accurate the location of the suspected fault, the less time staff spend repairing it.

Agree in principle though, is this extreme precision really needed?

[tobylane]

The lines with platform edge doors (Elizabeth, half of Jubilee) do need to be in the right place with 10-20cm accuracy.

[BigTTYGothGF]

Surely this has to also be a problem long solved by classical physics?

[TheOtherHobbes]

Track defects tend to be quite small, so yes, for track maintenance monitoring the extra precision is the difference between "somewhere in that rail, and you'll have to use expensive equipment to find out where before you start work" and "that's the bit that needs to be fixed."

[thaumasiotes]

> their accuracy is down to metres rather than centimetres. If you want to monitor track conditions, the more accurate the location of the suspected fault, the less time staff spend repairing it.

Considering the tracks are linear, I would estimate the additional time needed to locate a fault within two meters as compared to two centimeters at "negligible".

On the alternative assumption that the faults are too small for humans to detect and we just need to replace the affected track... I would also estimate the additional time needed to replace two meters of track, as compared to two centimeters, at "negligible". It doesn't actually take less time to cut out a specific 1cm strip (containing no visible indications!) from a piece of cloth as to cut out a 1m strip that includes the 1cm strip somewhere.

[bluGill]

Do they repair track cracks? I'd expect they replace the entire track section - both rails - as long as they are there. Get within however much they can do in a work shift and good enough.

[thaumasiotes]

So.... how would it be faster to do that if you knew the location of the fault to within 2cm instead of knowing it to within 2m?

[bluGill]

I don't know, that is why I'm asking.

My general thought as someone who doesn't know how rail maintenance is done is that rails should not crack in normal operation. If there is a crack that implies either the rail is end of life anyway and you replace it, or there is likely a manufacturing problem and you want to replace all the rail from that branch. Either way rail is manufactures in long sections (20 meters is my guess, but that is slightly educated guess that I won't stand by), so you would only need within a few meters to find the track section in question.

However I don't know how track maintenance is done. It is entirely possible that they grind/cut out the crack and then fill in (either cast in place, fill with weld, or just replace a a few cm) and in that case you would need to know within a few mm (though maybe inspection could find it if you are within a few cm).

Again, I am not a rail expert here. This is a place where I want to know and thus would like an expert to say. (though likely no experts are reading this...)