Welp, this is a bit frustrating. There is one rough spot in my track laying where my yard engine (an EMD NW2 from Broadway Limited) stalls, and of course it's inside the tunnel where I can't reach. My Heavy Pacific gets through there just fine, so maybe the answer is just to not let my yard engine out of the yard... but it's always going to nag at me.
It's frustrating on several levels because I spent a lot of time checking that particular stretch of track before I built the tunnel over it, and it worked fine at the time. However, I only tested with the Heavy Pacific as I hadn't yet bought the NW2. And I went to some trouble to leave access points to reach stuck or derailed vehicles in the tunnel, which turn out to be not quite large enough for me to reach that spot.
I might have to buy a track cleaning car.