They use “test” cars that have bright lights or xenon flashes pointing down. They take thousands of pictures of the track every night and store the images in a database that can match them up precisely so you can see cracks growing over time like a movie.

I also remember reading about an application of fibre optics where a long strand is placed directly under each rail. Pulses of light through the fibre are reflected at the points where axles press down on the rail and compress the fibre. Similar techniques can be used to detect accidents and (completely) broken tracks.