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.
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...)
