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- 30 - The momentum of the collision permitted the underframe of the older car to carry away the collision posts of the Highliner car. The sidewall and roof of the Highliner were lighter than the collision posts; the underframe of the "old" car demolished the superstructure of the Highliner and penetrated almost to the midpoint of the Highliner.

The unattached trucks of the old car absorbed practically none of the energy of 720. Since there was no electrical equipment suspended under the lead car of 720, the major energy attenuation occurred in the demolition of the rear car of 416 and the front part of 720. If the front trucks of 720 had been attached to the underframe, they would have resisted the telescoping and would have dissipated some of the energy. Therefore, the intrusion into the Highliner would have been less severe, and fewer passengers in that car would have been affected.

This deep penetration of one car by another can also occur in cars of identical strength and design. In an accident which occurred at Darien, Connecticut, on August 20, 1969, two commuter cars, each with a steel and stainless steel superstructure, collided head-on. The trains were both moving at approximately 30 m.p.h. In the collision, one car overrode the frame of the other and a deep penetration occurred, which destroyed almost one-half of the passenger space, as well as both the sidewalls, the roof, and the seats of the penetrated car.

There is little doubt that anti-climbing arrangements could be designed which would prevent penetration of one car by another. If overriding and penetration were prevented, however, any impact would be transmitted through the frames of all the cars in a train which has been struck, and this could result in much higher crash accelerations and in other unpredictable events. The collision of two Highliners at 16 m.p.h. without overriding resulted in generalized distortion of the car body and underframe but no gross injury-causing damage. This is efficient from an injury-prevention point of view but requires the costly replacement of the entire structure. Thus, although it may be possible to equip some existing commuter cars and rapid-transit cars with effective anti-climbing arrangements, the Safety Board believes that such changes should not be made without full-scale crash testing. Such testing could easily result in major changes in car design.

In general, the Safety board believes that the problem of specifying collision resistance for lightweight transit cars can be solved only through a substantial research and development program. Major gains in practical crash resistance probably cannot be achieved through small-scale changes in car design.