Mixing O and O-27?

The #1 cause of the issue you're describing is a loss of good electrical conductivity between apparently connected track sections.

Assuming you're using 3 rail O track, the 2 outer rails are typically Common (negative if you prefer) and the center rail is Hot.  So unlike 2 rail systems, there's no electrical polarity concern with loops that return back on themselves.

Solutions are to first:

1. Tighten the rail connections around track joiner pins. This can be done with a pair of pliers.
2. Then add more electrical drops along the track.  Typically using a distribution block to split each of the Hot and Common wires coming off the transformer so that wire pairs attach to the track roughly every 6 track sections.

Okay. That is good news. Thank you.

You probably have already observed this...  if you're inserting O27 pins into O gauge rail, they won't make good contact because O27 pins are smaller.

I'm not sure whether Lionel ever made a "transition section" to facilitate this scenario.  But you should generally shim up the O27 track to the height of the O gauge.  Make sure the alignment is spot-on, and then ensure good electrical conductivity by adding jumper wires-- at least one for ground and one for the center rail.  If this is a temporary holiday layout, you could use two lockons instead of soldering.  My $.02.

I've inserted a short  piece of bare copper wire under the insulator paper on the center rails of problem joints.

Jan

You show 2 transformers.  Have you split the track into blocks, at least 2 electrically separate sections where only one transformer at a time is in control of each section?  Is your drawing depicting a double track layout with 2 independent tracks?  You solved the pin size issue, the other is that O is taller than 027 and might need a shim at the meeting point.