Being that you need AC, it's either resistance, or you could use semi-conductor junction drops- diodes.
The short answer- get yourself some full wave bridge rectifiers. You short the + and - leads and and pass power through the ~ AC leads.
No need to rehash this same old topic https://ogrforum.com/...xed-voltage-bus-line
Here's a beefed up schematic version of the bridge rectifier diode method for voltage reduction.
If you use individual diodes, wired back to back, the picture at the bottom would be cut in half vertically between the 2 groups of diodes. Each set of diodes produce an AC voltage drop of about 0.7 volts. With the rectifier, you get double that.
https://ogrforum.com/...crossing-accessories
An option to those expensive high-power pots for lowering AC (or DC) voltage is the venerable string of diodes or bridge rectifiers. This has been written up extensively in OGR threads. Here's a photo showing a string of 4 bridge rectifiers hi-jacked from this thread.
The bridge shown above goes for about 30 cents each and a screw-terminal strip for about $1 each on eBay (free shipping from Asia). Of course available from US mail order sources for a bit more.
To your point about high-power, note that each bridge (4 diodes) drops the voltage by about 1.5V or about 3/4V per pair of diodes. You don't get the variability or adjustment resolution of a high-power pot, but at 8 Amps this amounts to a 12 Watt (Power = Voltage x Current) pot that has a fixed drop. I am confident that 3/4V resolution will be suitable - that is, you will be able to find a tap that works for your various crossing accessories.
In your application, it sounds like you need multiple outputs. What's nice about the diode method is you can simultaneously use different taps. I found this picture (but unfortunately could not locate the OGR thread) to illustrate how this might help you. This photo shows the starting voltage at 15V but yours would be 18V. The point is you are probably looking for voltages at 14V or below. If you allow soldering, you can configure the first few dropping bridges so that you get just the ~1.5V drop since you don't need the 3/4V resolution at the higher voltages. Then at the voltage range of interest you access the 3/4V resolution via the screw-terminals as shown below. The idea here is if you only have a 12-position strip, you can manage the terminals to advantage.
So one accessory can use one voltage tap, another accessory can use a different voltage tap, a 3rd accessory can use yet a third voltage tap, and so on. Or multiple accessories can use one tap if it turns out they work best on the same voltage. Note that if going with the pot method, you would typically need one pot for each accessory.
