LCS BPC2 ground wire

No other options that I can see per the manual - the commons are all connected on the ZW-L - you are correct that the U is the same as the track U -

Something like this would work - note that the manual says to use AC - so be careful which wall pack that you select 

Do not try to power it with a common ground, I suspect they have a reason for the warning in the manual!

I'd also rethink the brick recommend above, here's what the manual says.

I'd be looking at something like this: Arndt AM-121200A 12VAC 1200mA AC-to-AC Wall Adapter Power Supply, eBay: 112196256663

Thanks for the ideas. For the cost of the module, almost a $100, couldn’t have Lionel included a power adapter? Made is so much easier. 

Sure they could have, but they didn't.

I have a ZW-C and use handle D to power the replays on my BPC2.  From the Lionel LCS website it says : "An additional AC power source is required for relay operation. This external AC can come from your track power or an accessory transformer (sold separately)."  The manual says "Attach one wire to the Common/Ground/U terminal of your transformer and connect it to the POWER U terminal on the BPC2 unit. Do not connect this terminal to the outside rail."  I had emailed about this to Lionel, and I believe the response was I was fine using a channel of my ZW.  It's not that you can't have all the commons tied together (Which they are internally on the ZW-C.), it's that some people daisy chain their commons thru their track of instead of running back to the transformer.  Sorry, it was a long while ago so I can't get their exact reply.

I’ve long thought that Lionel manuals are way too cookbook and not enough technical information.