A 1950’s transformer (I think) it has 3 terminalsa A, B and U. I assume A is for track voltage , B is for accessory and U is common. Is this correct?
Lionel was very inconsistent about this. Terminal U is often the common. But on the 1033, for example, A was common and U the track voltage. What transformer do you have? You can get a definitive answer by looking it up at http://pictures.olsenstoy.com/searchcd2.htm?srt=7