That "weigh in motion device" is not necessarily an actual scale. Often it is a set of strain gauges mounted to the rails. This measurement, in conjunction with measuring the actual rate of acceleration down the hump, allows the hump computer to calculate the actual mass and friction coefficient of each car, which then allows a precise application of pressure at the retarders to cause a nice "Kiss Coupling" at the string already standing on the bowl track.

I do not know if actual load weight can be decoded from this data, which is what is needed for billing. Certainly, train crews do need accurate "gross trailing ton" data to correctly follow air brake rules. Inaccurate weight data (weighed by the shipper - perhaps falsified) some years ago caused a crew of a potash unit train to misjudge the number of retainers needed to be set before decending the Cajon. The resulting runaway was a long term disaster for the communitity at the bottom of the hill.

OPB