Some Warforged were modified to include within their bodies a rare, magical device that allows them to alter form. No one knows where this device came from, not even the Warforged. 

The Warforged must have the design of the vehicle in it's data banks, and can have up to 5 vehicles stored at a time. Scanning a new vehicle type into his data banks takes 5 minutes for a small vehicle (motorcycle), 10 minutes for a medium sized vehicle (car or buggy), 15 minutes for a large vehicle (bus or truck) and 20 minutes for a gargantuan vehicle (Semi truck or train). A gargantuan vehicle form can only be maintained for up to 20 minutes and the Warforged takes one point of exhaustion. (See Decent into Avernus for vehicle ideas and stats.