How can I simplify the electrical supply to my Christmas layout buildings ?

You could get a bunch of these.  Just run 16-18VAC power around the layout, place a unit need theitem needing to be powered, and hook the inputs of these into that feed.  Set the output voltage for whatever you need at the particular building.

https://www.aliexpress.com/ite...&ck=in_edm_other

There are led strips available that use 5 volts. I would suggest using a computer power supply. They have a 5 volt tap that would power many led lights. A single diode would drop the voltage to 4.5 volts for the signs, so you could use the same line to power the building.

As John H says there are 5V LED strips.  The relevance is that even Menards uses 5V LED strips in their buildings powered by 4.5V.  For example, Morton Salt:

In other words you can power a 5V LED strip with 4.5V to good effect.  Yes, it will be a bit dimmer than if powered by 5V.

So.  If you want to leave your 12V LED strip powered buildings as-is, then you would just need 2 AC-to-DC converter modules...one with output set to 12V DC, the other set to 4.5V DC.  Both would be powered by your existing 18V AC.  With the $2 new-customer coupon, the converter that RJR identified is reasonably priced.  Or there's the same converter on eBay for $3 (free shipping from Asia) but almost gone (whatever that means):

Or available on Amazon with presumably faster US shipping for a bit more coin.

In general, I'd support John's suggestion of an inexpensive 5V computer power supply with a diode to drop the 5V down to around 4.5V.  But since you say this is a Christmas layout where space/wiring issues are paramount, I figure the fewer 120V AC power cords the better.

Or. If you only have a few MTH buildings that you converted to 12V LED strips, perhaps you could re-do them to use 5V LED strips.  Then ALL your building would operate on 4.5V DC.  In round numbers, a single AC-to-DC converter module as shown and set to 4.5V DC output should power between 50-100 LEDs on a 5V LED strip.  So depending on how many converted buildings you have and how many 4.5V Menards/Miller buildings you have you might even get by with one AC-to-DC converter set to ~4.5V DC!  Your mileage may vary. 

Thank you RJR, John and Stan!  Great, this is perfect, I actually have a few of the Menards led strips I pulled from the York motel. The amount they had in there was crazy. I pulled over half of them out. Didn’t give it much thought that they are 5volt.

Just a couple more things to mention. The PC power supply also has a 12 volt tap, and also lightning fast circuit protection.

There are always a lot of helpful suggestions on the forum. So any feed back on this idea...?

Continuing to simplify the Christmas layout, I am thinking of setting these panel mount jacks into my plywood platform. I can then have short power leads on each building and plug them into the layout from the top.  All jacks neatly prewired under  the plywood layout with one or 2 leads out to transformer/ power source.

I will probably drill a hole size as close as possible so I can hot glue jacks in place somewhat permanently.

5.5mm x 2.1mm DC Power Jack Socket Female Panel Mount Connector Adapter

You might want to reconsider gluing jacks in place.  I have several Menards buildings in which a socket is mounted rigidly on the building, and a flexible wire has to come up through the hole in thetable