This is exactly how my signal detection system works. The thing I found is that you need really good magnets. At train speed, a regular magnet was not reliable enough. I switched to some rare earth magnets I found at Radio Shack ....
http://www.radioshack.com/product/index.jsp?productId=2102642
These magnets are under 1/4" in diameter and proved to be VERY reliable. Two tips I learned over the years for reliable operation - One, install the magnets on the trucks, this will ensure the magnet is always centered between the rails where the reed switch is, even in corners. I started by mounting them into a recess in the fuel tank, but in corners, the fuel tank swings off-center which made activation less than 100%. Two - try to keep the downward facing polarity the same throughout your fleet. I don't know if the reed switches develop a polarity or not, but I found two of my engines weren't being as reliable over the reed switches as the rest .... solved the problem by flipping the magnet over.
If company comes over to run one of their engines, I just stick a stack of two, three or four magnets to the uncoupler screw on their engine. Add enough magnets to get the stack as close to the rail as possible. The rare earth magnets will not fall off the screw ! As a last resort, the magnets can also be attached to the truck screw of the first freight car - they fit exactly into the recess of an Athearn truck screw hole !
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Lee Vallet tools sell rare earth magnets that are ideal for this. They come in sizes as small as 1/8" diameter and 1/16" thick. They are very powerfull for their size.
73
Bruce in the Peg
Miniatronics makes a package of reed switches and magnets. I think there are 5 of each in the package. They sell HO and N sets. The HO scale magnets are larger, because the car floors are further from the tracks and a stronger magnet is needed.
I used these on my subways, carefully positioned as "occupancy detectors" on stub-end tracks. They work fine for this. The magnets, though, are disks about a half-inch across and perhaps 3/16 across. They fit easily under the subway cars, but I don't think they'd work well under most diesels. You might be able to glue one somewhere in the pilot.
It takes an iron man to play with a toy iron horse.
I want to install a train detection system on my dc ho scale layout. I am planning on installing magnetic reed switches between the rails. My question is, what kind of magnets should I use, and will they fit under my diesel locks.