Here's a little background on the use of the letter "U" in electrical engineering (I got curious one day some few years ago):
It seems that in the early 1900's there wasn't such a thing as a well-regulated and stable voltage always available. Nowadays that problem is much less. But then there were fixed ratio transformers (well, we have those still, but aided by utility devices), with inputs like A, B, C on the primary and outputs on the secondary like a, b, c. (Different additional systems are used today, as well).
Then a customer could buy his own machine to adjust the voltage furnished to him. The inputs on this machine were w, x, y, z (I can't remember if 4 letters were reserved for input, but in those days a 2-phase 4-wire distribution system was fairly common despite the increased amount of wire). Then the output would be S, T, U, V, where if the output was wye, then T, U, V would be the 3-phase voltages relative to a neutral voltage on S. These could be rotary converters where the field strength could be varied to control the output voltages. Thus the letters T, U, and V came to be associated with a variable voltage.
Enter Lionel. On one smaller transformer I used as a kid, the 90w 1033, "U" was used to represent the variable voltage output. This is true of many of this group of small transformers. No matter how you wire it, you cannot create a dangerous voltage with a single transformer of these types. Interestingly, although I was an thorough reader of the instruction books and sheets, I never wired it the way the instructions say... I always wired its "U" post to the center rail. I also had no need of the lower 0-11v range; realized that with a single breaker the A-B combination at 5v was not protected; all this without a wiring diagram or opening the thing up. So I connected the A post to the outside rail.
Now I used an insulated rail to activate a semaphore. This rail conducted A to the outside post; I connected C to the center post for 15v for the arm; and then B to the other outside post for BC=11v for the lamp. In this scheme, the "off" break in the variable track output occurs at the end of handle travel where the output lowers to 5v. Turning the train off to stop it at the signal does not turn the signal off.
So it appears Lionel had applied "U" to a variable output following an old tradition. In that manner, the letter may have come to have a special significance. I think the ZW was made (even though it was too large to get a UL label) because kids were getting to have two trains, and its use reduced the risks associated with mis-wiring two transformers. At the same time, my feeling is that Lionel concluded that the special letter "U" would be better used to emphasize which wire should be the common wire between outputs. Or, as so often occurs today, the common wire between multiple transformers, and thus the wire relative to which all the transformers should be phased.
But the earlier transformer designs, where "U" identified the variable output, another post much be chosen. This is not always obvious, but for the 1033 its common among multiple transformers should be the "A" post.
The safety reason for this is extremely important. It is the single most important consideration in wiring multiple transformers. The touch voltage between any two posts or bits of metal track on the layout should not exceed 30v, per UL. This, with alternating current, corresponds to a sinusoidal peak voltage of 30 x 1.414 = 42.42v. A criticial voltage of 44v was long known from experience with DC voltage dating to the days of Edison. (The 48v mentioned above may be an AC peak determined from more recent research, but note that our whistle signal causes the max voltage output to have a DC component; I'd stick with 44 volts peak as max.)
The output of a postwar ZW is now 21v (designed as 20v when outlet voltage was 115v), plus the whistle output offset of 6v, or 27v total. To this must be added the power company overvoltage tolerance of 5%. Working this through, the possible output is just a fraction under 30v. Thus, the common post of any transformers phased with a ZW must be that post relative to which all its other voltages are higher and of the same relative phase (which is what "higher" means here).
Just my thoughts as to where "U" came from. --Frank
