An interesting use of loop detectors to detect vehicles on the tracks...

Gold Line Weekly Update and Photos

Loop detectors are those cuts, often a six sided pattern, that you see in the street, or on the freeway that detect and count cars. They detect duration of the change in the maganetic field from the iron in the vehicles passing over them. They are also used to actualize, or optimize traffic signal algorithms.

These "data points" on our freeways and streets, are often from those loops, and are used to create the traffic data flow rates/turned into estimated speeds, that are what is on your on-line live traffic web maps. Which took an awful lot of tedious work to identify and geo-code from Caltrans aerial photographs, or from what often turned out to be not that good of schematics, or that had to be calibrated from taking GPS measurements in the field, back in the early 2000's.