An intermodal transportation system having interconnected railcars adapted for transporting interconnected freight trailers where the freight trailers remain connected while being transported on the railcars. Individual trailers are assembled into trailer assemblies at an assembly area. Trailers containing freight are assembled into trailer assembles using specialized dollies. A preferred embodiment includes using a self-powered dolly having tires and mounting a fifth-wheel pickup plate and a ring coupler for engaging the kingpin of a trailer. Multiple dolly-trailer assemblies are connected together to form a trailer assembly. A self-powered movable ramp is positioned at an end of the railcar assembly and a tractor pulls the trailer assembly onto a railcar assembly by driving up a self-powered movable ramp and across the top of the railcars.