A new concept in the suspension of transit propulsion units has evolved from the emphasis on new developments, such as the San Francisco BART and the Northeast Corridor high-speed projects. Designed for the new generation of transit cars, battered and modified during operation at speeds up to 150 mph on Department of Transportation cars, this suspension arrangement has proved itself at speeds up to 150 mph. It reached fruition in time to be applied to the transit cars now being built for operation at 100 mph. Correcting deficiencies in the old design for lightweight, inboard journal trucks—motor and gear unit bolted solidly together, driving the axle through, and supported on rubber around the axle—yet retaining the principle of floating the motor in rubber to isolate it from rail shocks, this new propulsion-unit arrangement has successfully permitted car operating speeds to double, from 75 to 150 mph, in one jump. Propulsion units and lightweight trucks should never again be considered separately for speeds over 75 mph. They are married into a system and must be analyzed as a system. This paper tells how and why.

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