BAD PIECE OF FASTRACK

If you have a lighted caboose or other short illuminated car, rolling it by hand through the suspect track sections may reveal one or more bad connections.  Likely it's not the track that's bad, but rather loose rail joiner pins.

If you have a voltage meter or 12V test light, checking voltages with a load on the tracks can help narrow down which rails have poor connections.  Connect one test lead to the transformer and the other to the rails(s) with opposite polarity.  Then switch the test lead to the post on the transformer with opposite polarity from the first connected post and test the other rail(s).

There are several things you can do to improve the connections:

1. At the captured end of the pin, squeeze the rail from the sides with pliers at the very end of the rail,
2. Poor center rail pin conductivity can sometimes be improved by slightly bending these pins inward towards the mating pin, there's a video @RickO often posts when this question arises, showing the technique,
3. Add power feeders within the bad section.

Depending on how easy it is to remove the offending section(s) from your layout, you might also solder all of the tabs on the underside.  There are several metal tabs that are folded or crimped to make contact with the underside of the rail.  This isn't a reliable method of connection, especially if the track was bent during installation or walked upon.  Flip it over and flow a little solder around the tabs to make a positive electrical joint with the rail.

And if all else fails, just replace the defective section with a new one.  I've had some pieces of Fastrack that I just couldn't get to work for whatever reason.  I decided that for the cost, it wasn't worth the frustration.

Here's the video I was referring to.  The process is demonstrated by Mike Regan, formerly at Lionel.