hi, I understand where you are coming from, I might be 14 but I know what to do about storage, I would say about the track to unscrew it all and put it in a weather sealed package, or a rubber-maid like container, because if your son moves out i dont thing he would want to take an 8x8 plywood board with him, so mabey clean the tracks up and put them in a safe weather resistant area, now about your rolling stock and engines, for those I would say mabey try to recover the origonal boxes for them if you kept them, and if not then wrap them in bubble-wrap and once agian put them in a rubber-maid like container. If you have structers I would say to bubble-wrap those too and another rubber-maid container.
hope it helps
Beware of using bubble wrap or any thin flexible plastic sheet for long term storage! It 'outgasses' the solvents that keep it soft and flexible and can and will stick to the paint and cause it to come off or even corrode some metals! A better wrapping, believe it or not, is plain old brown paper like paper bags used to be made of, nothing with printing on it though. However, I recommend a soft cloth like a worn out white T-shirt or towelling and a wooden or fibre box. You can get these at good prices at craft shops, the sort of place that sells stuff for scrapbooking and dolls houses and flower arrangement.
I wouldn't use plastic at all, there's some pretty nasty chemicals in it and over the long term, who knows?
Also, if your trains have any kind of special electronics that have a start up sequence, like some MTH Proto 1 trains in O gauge, dont forget to include instructions in the wrapping otherwise they may never figure it out twenty years from now and you'll have forgotten. Also make quite sure you dont put away anything with a battery in it because for sure it'll go dead and may even leak. If at all possible put the engines away such that they are resting the right way up and put a THIS SIDE UP notice on the box as their own weight over the long haul may distort their bodies especially if there are temperature variations during storage.
Be aware that die cast metals containing zinc, most do, are susceptible to a nasty form of crystalline detioration caused by temperature variations, especially cold and humidity swings. If this happens its irreversible.
Last but not least, a process called electro galvanic corrosion occurs whenever dissimilar metals are in contact in moist environments. The moisture in the air acts upon the two metals making them act like a battery and in the process the least noble is corroded away. So for the maximum possible benefit I'd say wrap them in white cotton, then paper put them in a box marked THIS WAY UP and place those boxes in something that seals hermetically like a big Rubbermaid chest along with a generous handful of silicate moisture absorbing crystal sachets.
Thats probably a bit over the top BUT dont forget theres a good possibility that a quarter century down the road those models MAY have appreciated considerably in value.