Doppler Radar and traction control.

I often wondered how a locomotive's traction control system would know when the wheels are slipping if the speed recorder is attached to the wheels. Kind of knew that measuring resistance is one way, but had no idea that, with EMD's Super Series locomotives at least, a Doppler radar system was used to detect the actual locomotive's speed independent of the wheels.

While reading an article on the Class 66, the topic of radar was briefly mentioned, but it didn't expand on it.

Learn something new every day.