The idea is good but it puts the inspection of the train in the wrong place. Car men doing inspections in the yard find problems before the train leaves the yard and the problem car or cars can either be repaired or set-out. NDI testing of wheels can and is done on the fly. On the Portland sub we have a detector that uses ultrasound for the detection of wheel cracks, unfortunately, when active, it hits about every fifth train that goes by it. If there was a way to check knuckles for cracks on the fly, not a single train would get past the detector. In fifteen years of railroading I've been hit by detectors dozens of time and never once did we find anything wrong with the train. Every time I've had an actual issue with a train, it's been caught by a roll-by inspection from another train after having gone over numerous detectors without issue.
Scott