Cherry says:

It’s probably too late to share this, but sanding steel track removes what little protection its finish had and makes it way more susceptible to rusting in general because it increases the surface area of the steel in contact with the corrosive air. Using any abrasive is the same principle. Not sure if salt air as a rust cause makes any difference in that. It’s a BIG PAIN, but I’d replace all the track you sanded. If you’re filthy rich, buy stainless steel track and it won’t rust.

Spraying WD-40 on a paper towel and rubbing it lightly on the tracks can help prevent rust.

Evapo-Rust (widely avail in hardware & auto stores) will remove rust and may have some protective qualities as well after you use it. Just don’t rub it completely back off.

Basically you want to prevent the oxygen from reacting with the steel without making the track too slippery. I’ve heard that lightly rubbing used dirty oil drained from a car or truck engine on the tracks helps do that. I’ve never tried it but it sounds logical. If the oil is too slippery, rub some off. Or rub with WD-40 which will remove the oil and itself evaporate after a few days.