Because of your terminal arrangement, it sounds like you have the LW transformer which, as John said, is wired differently than most Lionel transformers.
With the LW, the A post should be connected to the outside rails of the Fastrack and the U post to the center rail. Because you have two (identical) A posts, you might want to try swapping them and see if that makes a difference.
BTW, getting 5 volts means you have the B and C posts connected.
You never responded as to how you were making the connections to the Fastrack but, as I said before, I would disconnect the wire leads from the Fastrack and measure the voltage between the two wires (one being connected to the A post and one to the U post) and see if you are getting full 18v voltage.
Gotta go - sorry - good luck !
