On Wed, 28 Feb 2007 22:19:38 +0000 someone who may be Dave wrote this:-
It is.
However, there are also other ways of inspecting track, because humans don't see everything. One obvious example is that humans don't see cracks inside things, until the crack reaches the surface.
One of these other ways of inspecting the track are high speed measurement trains, which run along the major lines to a regular programme. These are looking for many things, including measuring accelerations to see if there is a bad spot. The video is recorded so that if a bad spot is detected the video of the area can be looked at to see if there is a visible problem. It would take a long time for a human to study the video from just one day thoroughly and they probably wouldn't detect much due to boredom.