I'm not sure how rare, but the Lionel scale PRR T1 from 2000 only had one run, and because of the MTH lawsuit it may never be made again. While MTH has made a zillion (or more) of their version of the two prototype T1s, there are not many models of the 50 production T1s. And the Lionel version in really good.
Mine is also a bit rarer in that the four inside drivers, which were too wide and shorted on some switches, have been beveled so this is no longer a problem.