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Enabling Shared-Track: Technology, Command, and Control 45 Figure 4. Crash energy management (typical con- trolled collapse under increased load). ranging from low energy impacts for the hydraulic devices, through moderate, to high impact forces, which will be borne by the vehicle structure. Hydraulic devices. Operate using hydraulic fluid as a means of absorbing impact force and are already in widespread use in the United States on many LRV and rapid transit vehicles. Low speed impact forces (at speeds up to approximately 5 mph) can be absorbed (Zone A). Energy absorbing elements. Typically consist of volumes filled with honeycomb type material that absorbs impact energy by deforming during impact at moderate speeds (usually 1520 mph). Since many of today's vehicles are built with streamlined end caps, the volume present between the end cab structure and the front end of the car nose allows for considerable impact energy absorption and provides a small crush zone. In the event of deformation, the damaged elements will require replacement, but they are sacrificial components and afford more protection to occupants and vital components of the vehicle (Zones B & C). Deformable car structure. A design feature of European vehicles, these have end frames designed to collapse in a controlled manner and absorb any force left over after the hydraulic and crush- able devices have been exhausted. The collapsible frame members typically come into play only during higher speed collisions (above 1520 mph), and are designed to protect the driver's cab and passenger compartment in the event of a collision. Such an impact will of course result in more extensive damage and generate the need for substantial repairs, but the objective is to offer a higher level of protection to the passengers and crew, and to limit the propagation of the collision effects beyond the crush zone (Zones D & E). 3) Propulsion System DMU vehicles are powered by a diesel engine and come in three basic propulsion configu- rations: diesel-hydraulic, diesel-mechanical and diesel-electric, sometimes also referred to as a DEMU vehicle. Diesel-hydraulic vehicles use a hydraulic transfer case to power the train axles directly from the propulsion engine and diesel-mechanical use a mechanical transfer case, although currently the hydraulic transmission is the preferred of the two. Examples of these types of vehicles include the Colorado Railcar DMU and the Siemens Desiro DMU. Diesel-electric sys- tems, however, first use a diesel driven generator to produce electrical power and then use this elec- trical power to run electrical motors to power the vehicle (similar to a conventional locomotive). The Stadler GTW, Bombardier AGC, and Voyager all use Diesel-Electric propulsion. Diesel-hydraulic propulsion equipment generally costs less, requires less space, and is easier to maintain than diesel-electric. A diesel-powered car with an electric or hydraulic propulsion system will be the most probable system. This vehicle class is likely to accelerate far more quickly than a conventional locomotive hauled commuter rail train, a distinct benefit where track capacity is constrained.