The PD-4501 Scenicruiser, the most distinctive parlor bus design of the modern era, was the result of five years of effort based on a design by Raymond Loewy. Originally conceived as a 35-foot bus, Greyhound used a tandem-axle, 40-foot prototype called the GX-2 to successfully lobby for the lifting of restrictions against operation of 40-foot buses.
The first Scenicruiser rolled out in July, 1954 and was followed by 1,000 more.
The design was revolutionary in many respects. Seating was on two levels with ten passengers behind the driver on the lower level and 33 above in the rear, over the baggage compartments. Power was provided by two Detroit Diesel 4-71 engines driving through a fluid coupling. This installation proved to be less than successful and the 979 buses remaining were rebuilt with DDA 8V-71 engines and 4-speed manual Spicer transmissions during 1961 and 1962 by the Marmon-Herrington Corporation.
Information from the Bus World Encyclopedia of Buses – 1988