In a commercial world, almost nothing mass-produced is truly "rare". The older postwar products might be called "rare", but are not. This because toy trains were played with and many tossed out years ago- leaving fewer of them available today. BUT- we are in a hobby of shrinking demand.
Some exceptions are obviously those that were "mistakes" - with faulty stamping etc. - those are rare, but also have a very small market.
I actually saw a Lionel 2026 locomotive featured as "rare" and "vintage" on the Bay- when they were Lionel's low price line of O27 engines in their day- thousands were sold by every outlet including the chains. Another 2026 listed as : "WOW!!!" and "1952-53 ONLY!!!" on e-Bay posts.
Supply and demand is about all that counts. The supply is affected by how many were originally made (O27 more than O Gauge or Standard Gauge), and how many were played with to oblivion and/or were discarded.
The demand is affected by the age group that drove "train collecting" in the 1980's- 90s, but is shrinking due to having all that will fit in their space, or the (about) 1-2% of 70- 75 year olds who pass away each year. As well, an unknown number move to a place with no room for trains at all.
Plus, today, demand is softened because a lot of what is being released as "new" are really updates using new technology under the hood. But the shells might be made from older tooling. This means you can buy a new one, with better running and modern features- in quantity. Yes you might have to pre-order, or buy online since your LHS cannot carry $ 500- $2000 items that don't move (if the LHS even exists).
The bottom line- do trains for fun- and not so much for profit. Be happy if you can partially pay for your fun by selling off unused pieces. Sell those off to good home for whatever you can get, and be happy when someone else is happy.