In the old days, I used to watch SP trains occasionally stall on the very short but steep climb from Martinez to bridgehead. Soon enough, an SW1500 or two would be sent to push 'em on to the bridge. then they would drift back to Ozol, or wherever they had been working.

Also heard of (didn't actually see it) a couple of times, when the "Benicia Flyer", as some called it, would get stuck while going westbound the wrong way out of Benicia (westbound on that track was a very steep uphill grade). They would send an engine over the bridge to help pull them up and over.

I can understand a local switch job trying to drag too many cars back to into Ozol Yard. But don't quite know why through trains would stall, if they were within the ruling trailing ton limits of the assigned locomotives - but it was not that uncommon to see.