Well, as folks will tell you, I'm no expert.
But, I would start with the obvious. The power line connections that run to that section of track, shown in one of the last pictures, look really bad. I won't use the word sloppy because I know it is just temporary until you finish the track. To me, it looks like a short waiting to happen. It just may be possible that a wheel of your engine (on the inside curve of the track), or the inside edge of one of your rollers, is touching (or just brushing against) one of those metal power clips under your track, causing a short and the circuit breaker to trip, or if your engine has an e-switch, causing a momentary loss of power which causes the e-switch to default to the neutral position and stop the engine.
I would totally disconnect those power wires and clips, and go 5 sections further up that section of track and just use a pair of alligator clips to temporarily connect the power lines to the center and inner rail. Then start your engine one section in front of those clips, running in the direction which is away from the clips and towards your trouble spot, at a medium speed, and see if it runs through that trouble joint without stopping or tripping the breaker.
I have an engine somewhat like yours in the picture. It has really heavy trucks in the front and back, with what seems like a lot of overhanging and underhanging metal. This may be causing the problem.
Hope this helps.
Mannyrock
