The RailVac is exactly what is sounds like, a large vacuum cleaner on rails! Officially known as a Ballast Vacuum Extraction System (BVES), powerful suction removes material from between the rails and sleepers without the need to remove them and minimising risk to other infrastructure such as cables and pipes.
The RailVac is self-propelled whilst in operation during engineering possession though is locomotive hauled during main line transit (it can only travel at up to twenty km/h under it's own power). The RailVac was built, and is operated by, Railcare AB who operate similar machines in Sweden