As promised, here's a picture of the working train sensor we used for the Orange County Festival of Trees to control two 4561 Railway Express trains. When the train passes, the 5 sided flat piece is pushed inward. The short axle attached at 90 degrees (hidden from view) then pops off of the touch sensor (hidden under the 2 x 4 thin at the top and the 2 4x1 thicks. The long axle at the top is a deadweight whose purpose is to push the 5 sided piece back out when the train is gone. The 2 x 4 thin at the top serves as a stop to keep the short axle aligned on the yellow spot on the touch sensor. The 1 x black pieces under the two long black beams are also Technics beams. The two shorter beams, in concert with the longer beams, help provide a 1/2 bump offset between the sensor and the long beams so that the short axle length is exactly right (except for some filing for smoothness). You can attach this sensor to a RR tie with a rectangular brick. As mentioned previously, if you use new parts, the long beams tend to "pop" off the short beams after a while because they are a little slippery. You can over come this with glue, older bricks, or more bricks. There is room for exactly 11 thins between the bottom of the assembly and the bottom of the 5 sided brick, which makes this arrangement work great with the Railway Express.

Note that LDRAW truncates the wire from the wire brick so that it looks here like a short axle. The view above also shows how the touch sensor is offset 1/2 bump from being evenly aligned with the ends of the 7-hole beams.

-Ted Michon