When it was built in April 1954, the Pennsylvania Railroad's single class FD2 flatcar was the largest freightcar in the world, and although it never bore the name officially, it was known universally as the Queen Mary, after the giant ocean liner of course.

Its design capacity was a load of half a million pounds (250 tons, or 227 tonnes), and the car itself weighed slightly more than that, at 500,400 lbs. Its total loaded weight of over a million pounds was equivalent to that of the largest steam locomotives.

The intended use of such a giant freightcar was to haul heavy power generating equipment, specifically fully assembled turbo-generators for power plants. Most of the manufacturers of such heavy equipment in the United States at that time resided in the Pittsburgh, Pennsylvania area, on PRR tracks, and the railroad intended to capture such lucrative traffic.

Such generators are large as well as heavy, and to fit one inside the restricted loading gauge of the railroad, the car had to be a depressed center flatcar - a low platform (27 feet long, 8.23 metres) suspended between the front and rear trucks, 40 inches above rail level. At each end, the curving-up ends coming up from the load platform rested on the center pivot of a long span bolster which linked together two eight-wheel trucks, one pivoted at each end of the bolster. The trucks were reclaimed from the tenders of scrapped T1 class steam locomotives, and being built for the heavy load of fuel and water that a locomotive tender had to carry, were more than up to the task. Sixteen axles gave the car a tremendous load-carrying ability. The center body, pivot to pivot, was almost 74 feet (22.5 metres) long and the car overall was 124 feet (38.8 metres) long.

Only one was built, but in 1960 the car gained a dual identity, when another body was built to fit on the trucks and bolsters of the FD2. Classified FW1, this body was a well flatcar - a body with a large well, or hole, in the center. This opening was 42 feet (12.8 metres) long. Well flats were used to carry items that were narrow but tall, frequently items mounted on an axle, such as flywheels or turbines. The carbody was actually produced by United Engineering for use with one of their loads, and after they had delivered the item, the body was sold to the PRR.

It is not known what happened to the FW1 body, or whether it passed to Penn Central and thence to Conrail, but the FD2 body definitely did. In PRR service it was numbered PRR 470245, while the Penn Central gave it PC 766163 and for Conrail it was 766163 also. The car was withdrawn from service in the 1990s and donated to the Altoona Railroaders Memorial Museum in Altoona, Pennsylvania where it exists to this day.

Since then, of course, larger freightcars have been built, but none is as well known as the PRR's Queen Mary was among railfans. This may be because the Queen Mary was such a giant leap forward in comparison to previous designs, and featured in several Pennsylvania Railroad newspaper and magazine advertisements.