Well, model RR accessories are usually designed for 14-16V AC (the accessory terminals of a typical power pack).
If the lights are incandescent light bulbs, they will work with AC or DC. If they are LEDs, they will also work with AC or DC, but if those are white LEDs without any sort of protection diode in the circuit, the reverse polarity part of the AC power might exceed their maximum reverse voltage. Probably unlikely, but it would be better to power LEDs using a DC power, or adding a protection diode. Using DC, LEDs are polarity sensitive.