Over the years markets for commodities like coins and toys (including toy trains) wax and wane. In the 1960's U.S. coin proof sets jumped in value and everyone was scurrying to purchase them before the mint closed out ordering. Today, prices are about where they were in the 1960's (adjusted for inflation) except for the very rarest sets.
Commodity investing is very risky because so little of the investment value is intrinsic, most of it is perceived. In times of crisis the value of gold goes up. Has the supply changed? No. But the demand increases. When the crisis passes, the value of gold drops. When an investment becomes trendy (for example, emerging market stocks), investors will pile in trying to duplicate the high returns pioneer investors in the field captured. Valuations will be bid to prices far above what the market previously supported, then one day people will start selling to hold on to their returns. That starts a selling cascade and values drop. Exactly what we've seen in toy trains over the last 20 years.
Like it or not, for investment purposes, toy trains are commodities. Their value is subject to supply and demand, desirability, and whether trains are perceived as a hot commodity. Right now, we are in the midst of a selling cascade.
Several factors right now mitigate against toy trains as an investment.
1. The number of collectors is decreasing. (Fewer buyers.)
2. The supply of newer high quality trains has obsoleted the postwar period trains except as a nostalgia item to renew fond memories (I still have many of mine..).
3. The entrance of multiple manufacturers to the O gauge market in the 1990's exploded the supply of high quality trains, while the number of buyers decreased.
4. 1-3 above caused a collapse in the value of postwar trains except for those in mint condition.
5. Technological advances have produced rapid obsolescence of newer trains, sharply decreasing their value as well. We've gone from Proto 1, to Proto 2 to Proto 3, to bluetooth in 20 years. We have gone from Century Club to JLC, to Legacy to Vision Line, etc. The newest trains are amazingly realistic and keep improving. Bluetooth will likely simplify operations. For the most dedicated hobbyists who desire the most realistic model railroads there is a constant need to purchase the latest generation of models, resulting in a glut of the previous generation available, depressing prices.
6. The glut of supply and pressure to innovate has pressured the manufacturers into a cycle of adding new features and increasing realism at a time when there is a huge oversupply. The manufacturers have compensated by raising prices on the newest equipment.
7. Long term the manufacturers will face a significant challenge as train hobbyists continue to age out and are not replaced.
8. As with all commodities, condition largely determines price, and only mint or like items new can command any value.
9. This is just not a time to be looking at toy trains as an investment. Yes, there will be unusual exceptions here and there (the cast ES44AC), but these run the risk of being devalued when re-done with the next generation of technology.
My conclusion is buy what you like, enjoy it, but don't consider it as an investment.