Storing Trains

I don't think that's cast in stone. I keep my locomotives and rolling stock in their boxes and they've been fine for years. Out here right now it's 100 degrees. It's usually pretty dry, but since the pool is right next to the garage, there's humidity when it gets hot from the water evaporating.

That said, though, you wouldn't want excessive dampness coupled with cold weather as that would cause condensation (dry out those silica gel packets.)

Those are the figures used for most archival storage, particularly for paper items.  The boxes would suffer least damage under those parameters, the trains, track, and accessories have a wider latitude on temperature.  Generally, if you keep the area at a temp and humidity you would be comfortable with the trains will be fine.

I did not see any mention of how the original poster protects his trains before they go into the plastic storage bins.
If the trains are being wrapped in newspaper (very common) its a good idea to avoid direct contact between the paper and the train. Newsprint can rub off onto the train, and in rare instances, the paper can get stuck to the train.
I put an unsealed food grade plastic bag over the train, and then wrap in newspaper. If the train is too long for one bag, I use two, one on each end, and overlap them in the middle.

I have trains that have been stored this way for many years, with no ill affect.

I think humidity is more important than temperature. 

 I also think time alone is a bit of - well not an enemy, perhaps, but a nuisance.  I've noticed that:

- wheels and rollers "tarnish" even when stored in a box with desicant, etc., or on a shelf, etc.  I took down my Vision Challenger after it sat six months on the shelf last week and it ran terribly - really sick, barely responding, uneven speed, etc, sparks flying underneath it, etc.  Cleaned the wheels and rollers and (literally) good as new.

- grease hardens - not in a few months - but if more than two years I assume it probably needs to be renewed.

- Neolube tends to lead to wheels and rollers not tarnishing as quickly, but it too dries out eventually.

I'm not sure how well today's "hi-tech" engines would stand up to the storage conditions my post-war Lionels were subjected to...
My trains were unceremoniously stored in my parents' attic in their original boxes from 1965 to 1985 with absolutely no control over heat and humidity.  When I "retrieved" them for my two sons; I was amazed that my old U.P. Alco, N.H. F3, and Tie-Jector, all operated fine, as did the other accessories stored with them!