Fastrack switches wired for constant voltage

You need a good ground, but if you omit the ground, the aux power MUST share a common ground with the track transformer.

To elaborate a bit more, FasTrack Switches' ground terminals are already connected internally to the outside track rail (ground).  Connecting an Aux Ground to the switches provides a more reliable ground path back to the transformer, compared to relying on the track ground alone.  Like John says, sharing a common ground is important.  If Track power and Aux power (hot) for the switches both come from the same transformer, then the grounds are already common to each other (tied together in the transformer).  If Aux Power to the switches comes from one transformer and Track Power comes from another transformer, then the transformers must be phased and their grounds connected together in at least one place.

Do a search and read up on wiring Fastrack switches and be aware of some of the issues you may encounter.   My first few attempts were frustrating.  I did everything “right” but couldn’t get direct power to work.  There are some Fastrack switches that were wired incorrectly from the factory that might require rearranging some internal connections.  Wish I had known before I started so I wasn’t beating my head against the wall with what should be a pretty straightforward connection.