It's not the 'tracks' per se, as in the lumps of metal on the ground, but more frequently it's the design of the segment of the route. A piece of route might h

It's not the 'tracks' per se, as in the lumps of metal on the ground, but more frequently it's the design of the segment of the route. A piece of route might have been designed in the 1800s, possibly to a speed limit of 20 or 30mph, then been rationalised and 'upgraded' over the years to a higher speed, but that section of track will still carry limitations that are implied by the way it was initially designed to be, signalling may be too close together, or too far apart, junctions that are rarely used but remain for whatever reason add complexity and potential failure sites, tracks. There are of course some physical aspects of physically old track - shorter (~100m) sections of rail joined together have a higher risk of failure than the newer techniques of long (0.5km to 1km or more) single track pieces welded together, old wooden sleepers allow track movement that can result in delays and maintenance issues, whereas relative-new concrete sleepers, and really-new split-concrete sleepers are more reliable and permit faster and more reliable train operation. Tearing routes apart and redesigning the sections from scratch is difficult politically, so the UK's railway network tends to suffer from the fact that 99% of it was designed and built before 1880.