When using LED’s on your models, engines and rolling stock I paint the back side of the LED’s black. Say you glue the LED in a headlight mount, or any other light mounting that you just want the light to shine forward, LED’s give off a lot of light and if you don’t paint the back side of the light and mounting you can get a lot of light on the back side and it will light up the interior of what is back there. Thus light can come out of the engine where you may not want it and not look like it should, plus in some cases it can light up the plastic body and really look bad. Just something I do, keep the light shining where you want it to go.
Or just color in the backside with a black sharpie.