I was thinking the same as Elliot. Ducky, if you look at a piece of track closely, you will see what looks like a piece of cardboard under the center rail to prevent it from touching the metal tie. The outside rails do not have those insulators.
One of the pieces of track that was added in when you expanded the layout more than likely has a center rail touching and shorting the power. That will cause the clips to get hot.
Do as Elliot says to find the bad track or tracks. Then, you will be having fun again.
Thanks for photos of the trains. They look to be in good condition.
