Measurement trains filled with cameras and LIDAR
For example, in the U.K.:
https://en.wikipedia.org/wiki/New_Measurement_Train
https://en.wikipedia.org/wiki/Doctor_Yellow
LIDAR is good, but as another commenter pointed out, Ultrasonic Flaw Detection (USFD) is the gold standard for crack/flaw detection.
https://en.wikipedia.org/wiki/Doctor_Yellow