A serious safety problem with tractor/semi-trailer combinations is roll-over. The roll-over threshold of a combination could be increased by using active roll control to roll a trailer into a corner. The key components of such a system are a tiltable fifth-wheel coupling, actuators to apply a moment to the trailer axles, a source of hydraulic power, sensors and a controller. This active roll control system is entirely self-contained within the trailer, so is effective when the trailer is pulled by any tractor or used with a convertor within a train. An active roll control equipped trailer behaves exactly like the same trailer with a conventional suspension except for a reduction in the effective height of the center of gravity of the trailer. Increases in roll-over threshold of 20% to 30% are typical over a wide range of trailer weights and CG heights.