Joe,
That is a great idea and it would be very cost effective. I think however that the delta between pull strengths would need to be very high to compensate for inaccuracy when moving at speed. One would need to read the max value and that would be reached for a millisecond and therefore require a very high poll rate on the hardware. The delta would help smooth this out, but limit the number of identifiers greatly.
I was doing pro/con of ideas last night and I think I have settled on the method I am going to prototype. My original idea was to place the RFID stickers/tags on the trucks of every train and piece of rolling stock. At the intersection of each block I was going to place a RFID reader. This increased costs and resulted in a huge number of readers to network over large distance. My motivation for doing that was for yard sorting and firing events for specific stock under conditions tracked by the controller software. However, I realized I was merging the solution for yard sorting with block detection and the two could be mutually exclusive.
One of the reasons I went with O is because the inside of cars and trains is cavernous so adding custom electronics is no issue. So, one could supplement whatever decoder is in the board with a secondary board (Arduino micro) that has no job other than to broadcast the loco ID and its location over bluetooth. The Ardunio would route the RFID antenna to sit at the bottom of a pair of trucks on the train or tender. The problem is now flipped so the RFID tags can be placed in abundance on the track where ever needed to trigger location events or other events related to the train itself. Effectively, we are making DCC bi-directional now. : ) I imagine other possibilities like video feeds and fuel level reporting, but that'll be V2.
For yard sorting, I will still place RFID tags on the trucks of every car as well. I will only have to place RFID readers under the track in a few places to help with sorting cars at the yard throat.
