If you have the common for the Legacy connected directly to the outside track, it should go everywhere. I'm assuming all the outside rails are common in the layout, that's a requirement for Legacy. What is the ZW-I, I'm assuming we're talking about the ZW-L, right?
From the ZW-L manual, the "command base" connection is just the common side of the transformer. If you connect your command base there, it's connected to the common of the of all the outputs.