There are a number of reasons why the tracks were not destroyed. Probably the most significant is that it is difficult to do a good job when your army is retreating as you don't want to destroy the tracks too soon as it hampers your retreat, but leave it to late and the tracks get over-run.
Whilst ripping up the tracks is fairly easy (I've seen photos of a plough being pulled along the track destroying all the sleepers), it's far more difficult to destroy the route in such a way that it can't be rapidly brought back into service by re-laying the lines. Explosives can make a mess of tunnels and bridges but the majority of the track on the grade or in small cutting / embankments won't be badly affected.
Back on the bridges / tunnels front, again these have to be destroyed at the last minute. Whilst it was on the western front, the bridge at Remagen is an example of what can go wrong. The bridge was wired with explosives, but the explosives were fired too late (to allow a retreat across it) and failed to go off fully, leading to the bridge falling almost intact into allied hands.