On the UK rail network, multiple working is where two or more traction units (locomotives, diesel multiple-units or electric multiple-units) are coupled together in such a way that they are all under the control of one driver (multiple-unit train control). If the front locomotive of a pair in multiple has failed the driver can still control the rear locomotive for as long as air and electricity supplies are available on the failed locomotive. Many main-line diesel-electric and hydraulic locomotives are capable of running in multiples of up to three under the control of one driver