Originally Posted by banjoflyer: these appear to join the track together, not underneath
If you don't have one of the power prewired track sections you could try an item like this:
and substitute these track terminal joiners for the factory installed joiners.
I'm not really sure if this Atlas O gauge joiner would fit the Fastrack rail profile but American Models does sell rail joiners that do fit the Fastrack rail as I had to use one for a factory defect Fastrack curve section.
The prewired sections are by far the preferred solution.