Another Way to Install 
Infrared Detectors
Here is an alternative and somewhat easier method of installing NightScope™ Infrared Detector. The Product Manual and the step-by-step IR Detector Installation page show the standard method to install an Infrared Detector with minimal impact to existing track and scenery. This method does not preserve ballast, scenery, or ties, but requires no work from below the roadbed.
For applications where the Infrared Detector does not need to be hidden (detecting trains in a helix, for example), you can skip all of this. Simply drill a 3/8" hole and glue the Infrared Detector in place.
Install from Above: Step-By-Step Photos
Simply put, this method installs a NightScope™ Infrared Detector in a 3/8" hole drilled from above, and covers it with a very thin (0.005") styrene cover. To use this method between the rails, you will need to remove one or more ties to clear a 3/8" space.
In this example, we are installing an Infrared Detector into a 0.080" thick styrene sheet glued to 3/4" plywood.
Step 1: A 3/8" hole has been drilled from above through both the 0.080" styrene and the plywood. The Infrared Detector has been threaded into the hole — a nice snug fit.
Step 2: Here is the small scrap of 0.005" thick styrene (Evergreen #9009) which will be turned into a cap for the Infrared Detector hole. Two holes marked 7/32" apart and punched with an awl. We used the blunt end of a 7/32" drill bit to mark it as in Step 4a of the hidden installation method.
Step 3: Use a pin vise to carefully drill 1/16" holes. Hold the styrene taut to keep the drill from pulling and tearing it.
Step 4: A pair of nice 1/16" holes. Use a flat file to smooth flash from both sides of the styrene. You'll need to do this regularly from here on in the process.
Step 5: Carefully twist a round jeweler's file in each hole to enlarge to 1/8". Rotate counter-clockwise to avoid tearing the styrene.
Step 6: The blunt end of a 1/8" drill bit makes a good gauge to check hole diameters.
Step 7: 1/8" holes are done.
Step 8: Select one hole for the sensor and enlarge to just over 5/32". Carefully twist the round jeweler's file against the outer edge of the hole. Keep the hole round by working the entire circumference. The final check is the Infrared Detector as shown here.
Step 9: Now thread the Infrared Detector into the styrene/plywood hole from above. Finish by trimming the 0.005" styrene cap with scissors as desired and cement in place. Adjust the depth of the IR Detector and glue in place. Use a razor blade to cut the sensor shroud flush with the surface.