Generally speaking with classic transformers, in isolation, those designed to run a single train had the U terminal as the variable post. With transformers with multiple variable taps, the U terminal was the common ground.
Much of Lionel's documentation was focused on the simple layout, especially in their generic instruction sheets and it really didn't matter (inconsistencies can be found).
However, take a look at the advanced sections in this guide, specifically take a look at printed pg 43-45 (pdf pg 45-47) where it discusses common ground wiring. : https://ogrforum.ogaugerr.com/...sories%20%201954.pdf
The chart is expanded upon in the transformer section of the service documents: