Also, regarding the TW, the earliest produced models- such as the one shown in the schematics posted by @gunrunnerjohn , had an extra "b" post while all those produced 1954 and later lacked that post. "odd duck" is a totally appropriate name for this transformer due to, as has been stated and shown above, the constant, fixed voltage posts have an independent power source from the variable posts.
@Rex K congrats on the purchase. Replaced rollers and chord are a major plus- as is the warranty. When I purchased my first ZW, I was at the gentleman's house and he had a 275 watt and a 250 watt, the 250 watt had a new chord. I went with the 250 watt based on that alone as it was a sign that someone had repaired at least the chord and likely some other parts of the transformer. The 250 Watt ZW actually had a higher output than the 275 Watt Zw and ZW(R), 7-21 volts vs. 6.5-20 volt. Regarding extra protection for the transformer or trains, inline circuit breakers can't hurt, IMO, but that's up to you. Since you aren't messing with any sensitive electronics, it's not much or a worry for the trains. Regarding the transformer itself, and this goes for any transformer, make sure you turn it off when not in use. While the ZW(R) is supposed to "hum" less, if you find yours humming some electrical tape around the transformer below the binding post area can help
Finally, just an FYI, while you didn't say you'd be running any Marx or postwar Flyer- keep in mind that, if you do, these engines were made to receive 15 volts MAX! Thus, a postwar ZW could easily "fry" one of those trains.
Enjoy your ZW! Im sure you will- great transformer!
-Steve
