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46 Shared Use of Railroad Infrastructure with Noncompliant Public Transit Rail Vehicles: A Practitioner's Guide 4) Superior Car Braking Performance As mentioned above, and depending on material, construction techniques, and other struc- tural requirements, light passenger rail cars (defined as non-FRA-compliant DMUs and EMUs) tend to be considerably lighter than rolling stock that complies with FRA buff strength require- ments. Most are equipped with three types of braking systems; friction (either disc or tread or both), dynamic braking (this also can provide regenerative capabilities), and track brakes. Brakes can be actuated by air, hydraulic fluid, spring, or magnetic energy. Because light passenger rail car vehicles weigh less, their stopping performance using only disc brakes can be considerably better than that of an FRA-compliant design. But many modern light passenger rail cars also are equipped with additional track braking systems that provide increased stopping force in emergency situations and are much more resistant to reduced rail adhesion. The deceleration performance of track brakes can exceed 5 MPHPS and can stop a rail car going 30 MPH in approximately 130 feet (e.g., the Stadler GTW 2/6 vehicle). Track brakes are normally activated at relatively low speeds. However, because they are so effective the deceleration rate can be hazardous to passengers, and use should be limited to emergencies or on cars moving at very low speeds. Other operational benefits for shared use accrue from improved braking: A) Redundancy. A relatively high deceleration rate can be maintained in the event of failure of a single brake or system, compared with a conventional commuter rail vehicle. Furthermore the track brakes are a robust and extremely reliable fail-safe design that provides significantly increased braking force. B) Improved signal design. Traditional signal system design parameters incorporate safety fac- tors for loss of braking efficiency. The dependably high deceleration rate and overall system reliability found on these LRV vehicles may offer engineers more design latitude to design a train control system more suitable for passenger service, when compared to traditional rail vehicles. 5) Other Considerations A number of other vehicle-related factors need to be addressed in a shared operation. Coupler height disparity between FRA-compliant equipment and typical light passenger rail cars. Because of the relative difference in heights of buffing components at the end of each type of vehicle, the potential for vehicle override is increased, thus defeating the buffing mecha- nisms. This disparity argues strongly for fail-safe train control. Some light passenger rail cars can move their couplers to a safer position to protect pedestrians, but this is not possible with typical railroad couplers. Rail/wheel profile, which affects durability, noise, tendency to derail, and maintenance costs. The AAR wheel profile is standard for all rolling stock operated on the nation's general sys- tem of rail transportation. It is vital that any light rail vehicle introduced into a shared track operation conform to the AAR wheel profile. Shunting enhancement devices produce a magnetic field beneath the car that effectively acts as a shunt between the two rails to produce a block occupancy signal independent of any track or wheel conditions associated with standard rail shunting. Grade crossing collisions are a major concern for rail operations where grade crossings are present. Consequently warning system technology and mitigation techniques properly receive emphasis regardless of the type of rolling stock traversing the rail corridor. Reduced buff strength of light passenger rail cars may affect their survivability in a crash with a large high- way vehicle (truck or bus). However, their high deceleration rate offers a greater possibility of avoiding the collision in the first place or reducing impact speeds; and the energy absorbing features will mitigate collision impact with a large vehicle. Their length and higher acceleration