A class-I Railroad was having serious retarder control problems. Feedback circuits from a mechanical scale using a conventional strain gage was being fedback to the retarder control system. This was causing significant rail-car damage or added operational costs due to extra yard rail-car hostling.
An in-motion non mechanical rail-car scale was needed up-track from the retarders for significantly more accurate retarder control.
A scale which electronically senses the speed change in a free rolling rail-car once released over the yard "hump". Once this change in speed is measured Newtons equations relating mass, speed and acceleration are applied to accurately calculate the rail-cars weight. The rail cars weight was used directly in the retarder control system to properly control the rail cars speed through the classification yard.
Hall Effect sensors were used to magnetically sense the proximity of the rail-car's wheels. Optical sensors were deemed useless because of dirt and moisture that would inhibit the sensor from sensing car position. Two senosrs were used immediately after the rail-car release point and after the rail car was located completely on the yard "hump" glide grade.

These sensors were loacted 20 feet apart. Another pair of sensors were located down grade approximately 60 feet. Each set of two sensors would time the car between a fixed distance. A velocity was calculated from this time differnce. With the known initial velocity of the dropped rail-car and the velocity of the rail-car 70 feet down track, this differnce, in veolocity, track grade, distance and gravity acceleration constants were used to accurately calculate the rail-cars weight.