As for your application for model train ID...
As a general comment, I think you're on the right track to ride on to the coat-tails of an existing hi-volume/consumer NFC technology especially if you can get a shield and raid already existing code. Nothing profound here.
I'm NOT speaking from experience but IMO the problem with NFC technology specific to O-gauge would be parallel tracks. That is, if you are using RF to detect objects within, say, a 6 inch range, what happens if the device of interest is running a few inches away on the neighboring track rather than the one you're interested in? To be sure there are RF shielding techniques to "focus" RF energy...but it's not obvious to me you won't pull out all your hair messing with this!
I'd say the closest off-the-shelf "system" for model train ID is Lionel LCS which localizes to a specific track but it's not passive and not inexpensive. I think think it's a non-starter for a DIY project if ID is the application. There have been numerous schemes to print bar-code labels (passive) and optically scan passing devices. This of course gives you localized, per-track,, recognition which I think is a requirement. So again it comes down to exactly what you are trying to accomplish. An optical reader can detect whether the bar-code is being read forward or reverse to decode direction...and even sense speed. Then you can argue multiple NFC detectors can do the same with a dab of software.
I guess I'm saying it comes down to exactly what you are trying to accomplish. What's a must-have vs. a nice-to-have? How fat is your wallet? The same-old same-old engineering/cost tradeoffs. As they say, good, cheap, fast...choose 2!
I realize this is a somewhat wishy-washy response - but worthy of further discussion.
