Setting up question

Full 18volts to the track. 

No no just connect the Legacy 1 wire to the "U" terminal (outside rails) of the transformer. 

Yep. The black wire from the transformer should be connected to the outside rails. Take the 1 wire from the Legacy base and also connected it to the same place on the transformer. 

MartyE,

Maybe you can answer a question that has me baffled. I cannot understand why both Lionel & MTH say to turn transformers to 18V at the track? I've found that 12V works fine. I'm sure others question this also. Thank you!

In truth, while TMCC and DCS will run on 12V, you lose capability.  Cruise control requires some headroom to maintain the speed, so the locomotive has to reserve some voltage overhead to compensate for grades and track voltage variations.

Another issue is voltage drops in some areas of the layout.  Many, I'd probably venture to guess most, layouts don't have perfect wiring, so at some places in the layout you may have a volt or two drop under load. 

Finally, I know of very few TMCC locomotives that will fire the electrocouplers on 12V track voltage.