Carrying a skateboard while you board a train usually means holding it by hand or using a backpack with straps specifically for it. If you're on a bike, you'll need a Pannier Skaterack to hold the thing. The Snap Skateboard simplifies storage by sporting a foldable deck that lets you drop the entire board into a bag.
Billed as "the world's first folding skateboard," it appears like a regular board at first glance. The looks are a tad bare, especially with skateboards usually coming plastered with fancy graphic art, but they do seem perfectly functional, especially for cruising.
The folding deck is the only non-standard part of the Snap Skateboard -- everything else, including the trucks, wheels and bearings, uses parts you can get from any shop (or rip-off out of your brother's boards). The deck uses three aluminum panels that are attached using high-strength steel pins, combining to form a length of 32 inches. For storage, the thing folds down to a bundle that's only 14 inches long, allowing you to cram it into a backpack or a bike pannier.
According to the website, the board can be used to perform tricks, although the weight (6.5 pounds) from the metal probably makes it more suited for simpler riding tasks. It's sized for adult riders, but should be able to accommodate younger skaters who want the portability.
You can get the Snap Skateboard directly from the manufacturer's website. Price is $119.