Think I got it.
There was a junction that was showing the red/green arrow reversed. Now I've had that before, when a close examination showed that the track *actually* went just a wee bit to one side before crossing the mainline again and going far off to the other side. I consider that an illusion.
This however was a real problem, no crisscrossing tracks, just a bassackward arrow. Green to the right, train goes left, green to the left, train goes right....
I tried just breaking the junction apart and reassembling it, and deleting a single track segment, but the fix ultimately was to delete all three "legs" of the junction vertex and relaying those three tracks again.
One clue to the problem, deleting the left and right diverging tracks just happened to leave the junction lever behind and it did NOT go double red which is what you'd normally expect.