I think I have found the culprit.
I had the "9" 2D view open the moment the game wanted to quit because of a SPAD. My Q137 was running some 22 minutes late since I do not hunt for the absolute speed limit.
At around 17:29 when the SPAD occurred, Short Operation Service U876-C coming off the Salisbury Industrial Track had its lead engine's nose just past the dwarf signal protecting the entrance to Main Track #2 at Salisbury Jct.
U876-C was supposed to enter MT#2, reverse (in the siding at Yoder?) and pass Salisbury Jct 1730 on its way towards Cumberland.
If the Services Mode timetable is correct, U876-C should be out of the Industrial Track by 17:17???
I did check the entire Rockwood - Cumberland main to see if any other Services were in the foul, but couldn't find any.
Trailing X790 was at least two blocks behind, still running on Clear indication, despite being a light engine move.
Opposing Q138 appeared to wait patiently for its turn, scheduled to depart Rockwood at 17:38.
What to do next? Relinquish Q137 in the next attempt and let the AI engineer drive the train so I can follow it on the map and see what happens?
Edit: that's what I just did. I disembarked, waited for the offer to relinquish Q137, clicked Yes and hopped back on before it took off.
I am now standing behind the engineer looking over his shoulder, he obeys the speed limits, but drives like a maniac otherwise. No physics of a heavy freight train at all. So that's where DTG probably got its unrealistic timetable objectives.
I believe as soon as I sit down in the third man's chair, I am driving again, so better stay on my feet and see what the times at the objectives are.
Interesting, and I can finally operate the Horn Sequencer piston.
Engine sound change according to track speed, but no handles or indicators move. No track sound inside the cab.
Even more interesting, the cab gently tilts into the curves while the horizon stays level as it should!
Passed Hyndman objective marker at 16:32:30 or thereabouts, 3 minutes late.
Suddenly I am out on the platform of the first well car and cannot climb back on the engines ...
Passing Mance at 17:01:40 still 3 minutes late, I have been thrown back a couple more well cars, without any means of clawing my way back to the engines ...
In the meantime, U876-C was slowly crawling up the grade towards Salisbury Jct.
Around 17:14 Q137 passed Salisbury Jct, with me riding hobo on a well car. I could hear U876's SD40's down the hill, it is crawling at snail's pace.
At 17:32 I've arrived in Rockwood, the train finally stopped, and I was able to climb down.
Now, about U876, the presumed culprit: it did run the Stop aspect of the dwarf signal protecting MT#2, was it under track warrant to proceed to Yoder siding? The switch at Salisbury Jct seemed lined for the main track but U876 just crawled on.
Finally, there was the SPAD, when I was trying to board Q138 for the return trip to Cumberland. I think U876 either split the switch or overran the ABS Stop signal on the portal MP2162, probably both, its track warrant expired?
This is probably too complicated to report to DTG.
I have repeated the relinquishing of Q137, and am now sitting in the middle seat with a female engineer and conductor. When seated, the outside view tilts into the curves, which I think is wrong. Standing had it right, the cab needs to tilt.
Anyway, I am not lifting a finger this time and let the women do the driving.

The ladies are cheating, they use the passenger speed limit!