Train

Prohibited by all kinds of regulations, but still used by some cargo haulers because its cheaper than getting a real freighter. Basically, a tug is chained to the front of a cargo container with quick release cables, and burns all its fuel in the course of an hour or so to get up to high speed. At the half-way point, the tug detatches and allows the cargo to overtake, before recoupling at the rear end ready for a protracted period of deceleration.