I would not call them "rare", but hard to find and "highly desirable" based on resale prices are the MTH Premier M1A Abrams Tank flat cars.
Cataloged in 2007, the 20-98580 (European green) and 20-98603 (Desert) flat cars, as well as individual cars from 20-90361 and 20-90402 6-car set breakups, are regularly reselling north of $150 each on the big resale site.
Steve Glynn's MTH Product Reference Guide shows "Average" production for the individual cars and "Below Avg" production on the 6-car sets.
It will be interesting to watch how that resale price is affected by the new individual M1A Abrams flat cars and 4-car set releases from the 2019 v1 catalog.