I was using the PCB boards supplied by American Models for their cabooses and passenger cars. They consist of a rectifier, 470 microfarad/35-volt capacitor, and 2 or three incandescent bulbs. I have replaced the bulbs with up to 6 LEDs in parallel, each with a 470-ohm resistor. The LEDs come on at about 4 volts and reach acceptable brightness at 7 volts, but the light varies with input.
Using a buck-boost regulator appears to offer steady lighting, but the track voltage would still have to reach 7.5 volts to light up a 12-volt LED strip. The only advantage there would be the constant lighting effect. Would a better approach be to use the buck-boost regulator with the 5-volt LED strips so that they would light up with 4-5 DC volts applied to the track, or would that still pose a heating problem with Legacy 14 volts AC applied in those rare--or accidental--cases?
