[cweagans]

If you wanted to avoid the clock thing, you could probably do something like this.

Have two rows of sensors set up like this:

    * * * * * * * *

    *             *

When both of the second row of sensors are high, read an 8 bit binary number from the first row of sensors. This allows you to have up to 255 different IDs with 10 magnets per train. First row is data, second row is alignment. Fast moving trains might have issues with this kind of setup though.