Abstract: A system and method for expanding a frame of a vehicle at least after partial assembly of the vehicle along an assembly line. The frame includes opposing first and second side rail assemblies that are, during at least initial assembly, in a first configuration that comprises, for each side rail assembly, a removable first insert mechanically coupled to front and rear sections of the side rail assembly. The vehicle can be selectively separated and expanded to a second configuration via the first insert being detached from, and a second insert instead being attached to, one of the front and rear sections. The first and second inserts can be coupled to opposing ends of a rail sleeve that is positioned between the front and rear sections. A supplementary floor pan that can be coupled to additional seat can positioned over the rail sleeve.