I set them either low in the vestibules or in the "bathrooms" in the sleeper cars. Many of my passenger cars have window shades that are pulled partially down or completely closed while other windows are fully open to view a seat or a passenger or two in the seat. I put the buck-boost in a place where it cannot be seen through a window with the shades down.
The example for which Ray provided a link will provide a constant voltage range of 1 to 17 volts. Less than 5 volts is too little voltage to light up the LEDs, and I found that a setting of 7-8 volts worked well. The light is not so bright that it is obvious when the room lights are one, and at night the amount of illumination is perfectly natural--and the body does not "glow" with too much light shining through when the train is running fast.
Heat damage is unlikely because the LEDs are extremely efficient and give off very little heat. In those cases where American Models had the PCB board suspended near the ceiling, such as the Budd cars with their detailed interiors, I fastened the LED strips to the PCB board itself so the the light came prototypically from the ceiling. In the streamlined and heavyweight cars, I used a wooden support running from one end of the car's vestibule to the other vestibule and glued the strip to that support. The two ends of the support were heavy cardboard with the image of a real vestibule door glued onto the side visible from the vestibule windows. A viewer looking into the car from one coupler or the other could only see the paper/cardboard vestibule support as if it were part of the car interior. The wooden support was at ceiling height so that the illumination came from the ceiling rather than from the floor where the original incandescent lights were.
I did not fasten the LED strip to the ceiling because of concern about heat. I used either the PCB board in the Budd cars or the wood strip held up by the cardboard vestibule wall because that allowed me to remove the body from the car to make any adjustments without having to worry about stretching wires connected to the top of the body. One could attach the strip to the underside of the body, but it would mean limited movement whenever the body were removed for maintenance--and the longer wires would probably be visible through the windows.
