As noted by many already, old train values can be all over the place. Very few appreciate in value and if you look against inflation many that are truly rare and worth much more now than when new are actually static or have held their value and little or nothing more.
If you collect model trains for investment, (brass, special runs, commemorative specials, etc), then you might not be a model railroader at all, but just a collector who chose trains to invest in.
If you are a true model rail and are looking to buy trains for running, then mint, non-brass stuff from the 50’s, 60’s, 70’s and 80’s, and I’m talking motive power here, is pretty much garbage by today’s running and operational standards. This is not to even mention true scale detail related appearance being sub-standard by today’s critical eyes. I doubt if time will ever favor these old models. Much the same for rolling stock of this period, too.
I wish I had bought Varney’s “Old Lady” back in the day when AHC sold them in their 3-4 page adds in MR as effective surplus from Varney’s closure for under $30.00, but I would still have a terrible runner, operationally, unless I dumped money into upgrading her. But… that is nostalgia.
MR stuff can certainly be or become collectable, but that doesn’t mean it is rare or valuable.
However, an old Varney or Tyco train set from the 50’s or 60’s pulled out of a rummage sale with its “Little Joe” dockside, 4 cars and oval of brass “Snap Track” is just the thing to infect a young person and make a new or budding model rail who is hooked for life. That is what is rare and valuable about old trains.
Trains are not for collecting, for the most part, but for running and enjoyment by the model rails who love them.
Richard