A semi-trailer is a carrier in which the chassis and container body are one integrated unit, unlike a typical chassis and container combination. In general, trailers have wheeled-axles at the back that supports the trailer while stationary or in-transit, and a landing gear in the front that supports the trailer when stationary.
Trailers are transported via train on flatcars that are referred to as "Trailer-on-Flatcar" (TOFC).
When a trailer is connected to a tractor (common trailer prime mover), it looks like a standard truck/chassis/container setup combination:
