How many switches on your layout? And to be clear, you're referring all the lamps dimming when one switch is thrown?
If the aux power wiring to multiple switches is a daisy-chain, then it seems switches closer to the transformer will see less drop and might actually get brighter if some kind of boost method is used!
Is conversion to LED a viable option? Note that this would likely require more than swapping out the existing filament bulb for a basic LED equivalent. The LED must have some kind of voltage regulation or energy storage (capacitor). Why? Because you will still see the same voltage drop from the large momentary solenoid current! So there must be circuitry at each switch which I suppose could be considered a bother to hook up.
If attempting to "boost" the voltage at the transformer (rather than regulate/smooth the voltage at each switch), my guess is converting to all DC operation might make for an easier circuit whether staying with lamp or converting to LED.
