Third Rail Mobility, a start-up focusing on mobile power accessories, on Wednesday released its debut product: Slim Case for iPhone 4 with Slide-On Smart Battery. Ammunition developed the total design solution for the Third Rail System, creating the overall industrial design and and assisting in the development through to manufacturing.

The Third Rail System is the first protective case for smartphones with an optional, modular slide-on battery. The system, which can be purchased separately or in a bundle, includes a Slim Case to protect your iPhone 4 and a 1250 mAH Smart Battery that you can slide into place whenever you run out of juice. The removable battery can be stacked up to six deep for additional power and can charge other (non-iPhone 4) devices using a micro-USB.

Initial product reviews have been positive with CNET dubbing Third Rail as “one battery to power them all.”

For more information on the product, visit Third Rail Mobility.