before they too were replaced with diesel-electric units, such as the LRC train. Amtrak continued to run Rohr Turboliners until about 2003. JetTrain Another change that has taken place over the last few decades is the widespread use and standardization of head end power (HEP) to provide electricity to the rest of the train for running the environmental controls and entertainment systems. Since these power requirements are fairly steady, even while the train is parked, it is not uncommon to use a separate engine just for this role, highly tuned to these operational needs. The JetTrain concept expands on this idea by using the same HEP engine to provide motive power during low-speed operation, bypassing the problems with turbine fuel efficiency at low power settings. The JetTrain concept uses only this engine while it is moving about the station area and switchyards. Once the train has left the switchyard, the turbine engine, a Pratt & Whitney Canada PW150 (called the ST40 in this role), is started and the train accelerates to operational speed. The turbine engine is adapted from helicopter service, having the benefit of over 50 years of development in that service to improve its operational efficiency and reliability. It is adapted to operate on regular locomotive diesel fuel so that no special fueling facilities need to be built to introduce the turbine locomotives on existing railroad lines. The diesel engine alone can run the train at speeds up to when empty, and the turbine increases that to . The two engines are geared together in a single gearbox which powers a generator to provide power to four traction motors, identical to those in the all-electric Acela Express. The generator is the motor from a TGV train, run in reverse of normal operation where instead of being hooked to electric power and producing rotation, it is rotated and produces electric power. The major advantage to using the turbine for the high-speed portion is its small physical size and light weight. The turbine engine is about the size of a common office desk and weighs only , while a conventional diesel motor of the same power is about 5 m (16 ft) long and weighs as much as . Using the turbine lowers the weight of the power cars; the JetTrain power car is and had an unsprung weight per axle of . This compares to the widely used EMD