Well, I have noticed through a lot of posts that Geometry Cars are in use by many roads. Now, how do one of these cars work? Do they have lasers that scan the track or are there special things that look at the track and see if its warped, etc.
Earlier cars used measuring "swords" that made physical contact with the rail. The cars used these, plus accelerometers, to measure gauge, cross level, alignment (right and left rail), and "twist" (a rate of change measurement).
Newer cars use non-contact measuring devices such as lasers. A digital record is made of each run, along with a physical strip chart. Off-board software is typically used to analyze the data, but there are some real-time packages that can analyze information as it's recorded and spot combinations of measurements that may pose derailment risks, even if each individual measurement is within specs.
Check out Plasser's Web site for more details.