Power pack question for building lights.

It's called a post-war transformer.   You can set the voltage to whatever you need.

Not quite sure what you have.  If you have the Just Plug system with Light Hubs (that have adjustable brightness), note WS suggests powering these with more than 12V AC. 

If you have the pre-built WS buildings with built-in power supply, that's a different animal.

Also, if you can use a DC supply (rather than AC) you will find more options at substantially lower cost.  For example, for less than $10 (free shipping from a US seller), this 90 Watt DC supply has a switch-selectable voltage output - one choice being 24V DC.  Much more compact than the 100 Watt Z-1000.

Stan I am using the Woodland Scenics power supply pack for the WS buildings. I am referring to non LED, 4 Lionel station platforms and older 50's Lionel passenger and freight stations, and MTH buildings. I also have a Riverleaf Model building with lights  (recently purchased). My mentioning the just plug system might have confused some. Sorry. 

John I thought of using a postwar one, but I fear that I might accidentally hit the handle and set it too high and blow out all the bulbs.

If you're powering incandescent bulbs, 12V DC provides the same brightness as 12V AC.  That is, 12V DC and 12V AC provides the same "RMS" voltage which determines how hot the tungsten burns so to speak.  Of course if this 12V AC is also supplying accessories that require an AC voltage (like vibrator mechanisms in rotating searchlight, wig-wags, etc.), then you get no joy with a DC supply.

12V DC power supplies are far more readily available and inexpensive than 12V AC supplies.  As to any apprehension about driving incandescent bulbs with 12V DC instead of 12V AC... think automotive!