Even if the track rails do not look rusty, they still can have oxidation on them. It can be enough to prevent good electrical conductivity to your car's pickup shoes. And, just because a track set is bought new does not mean the rails are perfect. New sets can have been in the distribution channels for 6 months or more prior to being put into retail stock. The time and different storage conditions can all lead to oxidized rails. The good news is that the problem doesn't take much effort to fix. All you need is a fine drywall sanding sponge and a microfiber dust cloth.