On Wed, 3 Dec 1997 20:19:13 -0500 Engineering Department
<imagelogic@IBM.NET> writes:
>> For a model train, I would suggest either a hall effect sensor
>> or a phototransitor..
>
>What about a bar code?  I hear real trains use
>bar codes to label cars.

They tried that, but the bars often were obscured by dirt.  So they use
RF "tags" now.  The train-mounted part modulates a serial number onto the
signal reflected from a track-side radio transmitter.  This is different
from the anti-shoplifiting tags since it is necessary to uniquely
identify each car.  The anti-shoplifiting tag detector only identifies if
a tag is present or not.  The presence of a train on the track is of
course detected by the wheels shorting the two rails together.