Step 1. Assembling all the stuff

Two 2x4 plates
Technic wire (1/2)
Base cut from Lego packaging material or other 0.020" plastic sheet
Reed switch element www.digikey.com part # 306-1077
Magnet www.scientificsonline.com part # CR30822-73
Diagonal cutters
Dremel tool
Hobby knife
Glue

Step 2. The sensor pieces

Step 3. Prepare the bottom plate

Cut a 1/8" wide gap at the end of one plate and trim 0.020" from the
three center circular projections.

Step 4. Drill

Drill two small holes in each side. Between 3/64" - 1/16" diameter.

Step 5. Prepare wire

Cut the Technic wire in half. Separate and strip one piece as shown.

Step 6. Install wire

Poke the two stripped ends into the holes and glue the wire to the underside
of the plate. 

Step 7. Install base cover

Trim a piece of the thin plastic sheet to fit into the underneath of the plate
and glue in place.

Step 8. Install reed 

Glue the reed element in place , trimming the wires such that the overall length 
becomes 7/16". Observe the warning about cutting the reed leads. Trim the 
Technic wire.

Step 9. Connect reed

Solder the reed wires to the ends of the Technic wire. Do not take too long as the 
heat will affect the base plate.

Step 10. Prepare the top plate

Remove the center circular projection altogther and cut a small segment from the 
inner edge of the other two.

Step 11. Complete the assembly

Ensure the top plate fits on the bottom plate, ie; there is enough clearance for the 
reed. Then glue in place.

Step 12. The magnet pieces

Step 13. Prepare the 1x2 plate

Remove the center projection from the underside of the plate. 

Step 14. Glue the magnet in place

This completes the assembly.

You may now install the sensor in the track as shown in the picture. Attach the 
magnet to the underside of the train so it passes within 1/2" of the sensor. Note 
that the long axis of the magnet must align with the long axis of the reed.

The sensor can be connected to an RCX. Configure the input as a touch sensor.

It is possible to build sensors with the reed at right angles to the position shown 
and then mount the corresponding magnet aligned with the long axis. This can 
enable you to detect the difference between two different trains or train cars.