I'm sure many of you already know this but I thought I'd post it for the few who may not. If you are using Atlas HO-scale Code-100 snap-track in your layouts as I am, standard LED holders available from most electronic component outlets fit perfectly (diameter-wise) into the bumper holes. I am installing a pilot light on one of mine and have a choice to trim off the excess plastic under the track or sink it into my foam layout base. There is also plenty of horizontal room under the bumper for the series resistor. That's it.