That doesn't explain the very sudden, very violent sounding/feeling transition that occurs, on SD-50's and SD-60's, where the loco doesn't drop it's load, then slowly builds it back up, but suddenly the loco "pops" into transition, with a bang, and the revs of the engine don't alter significantly, nor is there a drop in TE or it's accompanying drop in speed, as the loco unloads...........
The "kick in the pants" can be felt clearly in the seat, as the loco quite suddenly transitions, and maintains a full load while doing it.
GN asked "can it happen"? Yes, it can, does and has. The reasoning behind the staggering of the transition speed by a few tenths of a MPH between locos, over a few MPH total range, is supposed to eliminate that. Imagine starting a train, of grain, and four SD-40-2's are pulling in the 8th, approaching 24 MPH. Suddenly, they all transition at the same time. All four locos simultaneously drop their load, as transition occurs. The dropping of load causes the cars to roll against the power, bringing in a good piece of slack. Those ponies suddenly come back to life, and 4 big locos then get a solid yank on the train, as they load their way back up to 900 amps. that slack that reached 10-15 cars into the train, is suddenly ripped out. Anything that is even microscopically compromised is now subjected to a violent DB pull in excess of the continuous DB limits for that piece of equipment. Ker-POW goes the weakest link. The locos are pre-programed with a specific transition speed, withing a very tight range to eliminate that from happening.
The other problem, with locos having the same, or very close transition speed, has happened to me. Pulling a train on the Riverline, for ConRail, I had the pleasure(?) of a solid GE consist, of 4 B units. As my train reached transition speed, the locos all seemed to transition at the same time. The sudden loss of tractive effort immediately reduced the speed, and the locos made reverse transition after failing to make a successful forward transition. As I got to that speed again, the same thing happened over and over. It took cresting a grade, before I could build the speed to overcome the locos, that "couldn't" transition, due to the weight of the train, and the amount of time the transition took. It made for a very long, and very frustrating trip.