Don, this would be a fun and relatively simple project -- assuming you've overcome the initial challenge of learning how to use an Arduino. Both the TMCC control and the RFID communication can be done via serial ports, so the interface is really easy. Back in 2015, I played with the ID-20 RFID reader. It has a good range, so you can put it in a trackside structure and still be able to register RFID tags that have been placed in motive power as it passes by. There may be something even better out now, but I was very pleased with the ID-20 and I believe it's what Professor Chaos uses on his layout.
Note that this isn't, and probably shouldn't be a "shield", because it will be simpler to keep the RFID readers trackside and keep your Arduino in a more convenient location. You'll just need a few wires for power and the serial signal.
The challenge I see is that you propose placing RFID readers at every block -- which presumably means a bunch of them. This was my original idea as well, with my autonomously-controlled layout. However I soon realized that if I can just determine where each train is initially (perhaps using a single RFID reader, or by typing in a starting position for each train), I can use any sort of occupancy detection (such as an isolated rail section) and "know" which train is tripping it -- by virtue of where it was before and which direction it was heading. On my own layout, I "register" trains at the beginning of a session (using an A/N display and rotary encoder) -- defaulting to the last-known position so it's usually trivial -- and then let my software track them using about 50 isolated track sections from that point forward. I wound up not needing the RFID reader at all. Another approach would have been to register trains by driving each of them past the RFID reader -- but that would have been a lot more effort for the operator than the solution I ultimately chose.
If you decide you still want to use multiple RFID readers, you'll need to figure out how to get them all to talk to your Arduino. Serial readers might be possible using Software Serial and a Mega, or you could try to find readers that used a different communication protocol such as I2C or SPI.
Please keep us posted on your progress!
