How much weight a freight car can carry is detemined by two factors: the structural design of the car and the size of the journal bearings on the axles.  Of course, the quality and condition of the track can affect how much a car can carry too.  Factors such as rail size, cross tie spacing, and bridge strength figure into this; but even heavy rail can't support much weight if it hasn't been properly maintained.

Most track can accomodate almost any load, if the weight is spread out over enough axles; but this principle doesn't apply to bridges. A bridge is designed to accomodate a specific maximun weight, referred to as its Cooper's Rating. The number of axles required to support a specific load on a rail line are determined by the track's maximum axle loading.

The maximum axle loading permitted for unrestricted interchange in the United States is currently 65,750 pounds, although there is a move on to raise this to 71,000 pounds, and some cars are built for a maximum axle loading of 78,750 pounds. The heaviest 6-axle locomotives have an axle loading of 72,000 pounds, but some 4-axle units have a higher loading.

Journal bearings have been manufactured in standard sizes which have increased over time. Listed below are most of the standard sizes.  The nominal capacity is the gross weight for a 4-axle car minus the weight of a typical car, usually 60,000-65,000 pounds.

At the beginning of the Railway Age in the United States, most railroad cars rode on two axles; but by the middle of the Nineteenth Century, most new cars rode on two 4-wheel trucks, and this has remained the standard freight car configuration for over 150 years. The one major exception has been intermodal cars. In the early 1980's there was some experimentation with 2-axle cars most notably the TTOX / TTUX Front Runners; however, the production of these cars didn't last very long. For the past 20 years, the most common intermodal designs have been various articulated or drawbar-connected platforms that ride on standard 2-axle trucks.

Throughout the twentieth century, heavy-duty flat cars have been frequently equipped with more than four axles; but these cars have typically been built in very small batches and have not been very common. The most widespread use of six-wheel trucks under freight cars took place in the early 1920's when several of the Pocahontas Region railroads purchased at least 2300 100-ton coal gondolas. While the C&O, N&W and Virginian had excellent mainline track, neither their branch line tracks nor the axle bearings back then could handle 100-ton cars on only four axles. This was an experiment that has never been repeated on a large scale.

The next large purchase of 6-axle freight cars occured in 1952 and 1953 when the U.S. Army bought 755 flat cars with a nominal capacity of 100 tons. Most of these remained in service into the early 1990's, when the AAR's 40-Year Rule caught up with them. Between 1981 and 1992, the Department of Defense supplemented its heavy-duty flat car fleet with almost 600 6-axle 150-ton cars. DOD also has a small fleet of special-duty flat cars with six, eight, or twelve axles.

During the 1960's railroads looked for ways to lower their costs and, in some cases, to pass those savings on to their customers; but it wasn't always easy. Among the very first of these big cars were the Southern Railway's Big John covered hopper. The Southern felt that a big 100-ton covered hopper would be a more efficient way to ship grain than a 40- or 50-ton boxcar, so the railroad developed its Big John car.

The first 75 Big Johns with a volume of 4,713 cubic feet were built by the Magor Car Co. in May 1960. The Southern attempted to implement new lower rates in August 1961; but the Interstate Commerce Commission blocked these tarriffs on the grounds that they could drive truckers and other railroads out of business. ICC hearings began on 8 January 1962, and the issue went all the way to the Supreme Court. The Southern won, and the new rates finally went into effect on 11 May 1963. By this time, the railroad had acquired an additional 500 even bigger cars, with a volume of 4,948 cubic feet.

What followed was an explosion in the size of freight cars. Some cars didn't look much different, although they had a higher load limit; but many car types got bigger, and in some cases, a LOT bigger! Box cars which had been 40 or 50 feet long became 60, 70 and even 85 feet long. Other cars got bigger too, but in most cases their nominal capacity seldom exceeded 100 tons. (During this time, the 125-ton 4-axle truck was introduced; but relatively few 125-ton cars were built because most rail lines could not handle them.)

In September 1963 the Norfolk & Western completed a 146-ton, 6-axle hopper car at its Roanoke Shops. N&W 76950 rode on a pair of 6-wheel Buckeye trucks and was almost 70 feet long. It had a light weight of 101,300 pounds, a load limit of 293,200 pounds, and a volume of 5460 cubic feet. Unfortunately, the car had two drawbacks: its size was incompatible with the railroad's rotary car dumpers at Norfolk; and its size only enabled it to haul 133 tons of coal, 13 tons less than its capacity. It was retired and scrapped in 1968.

In June 1964 Pullman-Standard constructed a 135-ton, 6-axle covered hopper for the Atlantic Coast Line. ACL 500000 made extensive use of stainless steel and rode on a pair of 6-wheel American Steel Foundries trucks. The car had a light weight of 93,500 pounds and a volume of 5,006 cubic feet. While its trucks gave it a theoretical load limit of 301,000 pounds, its official capacity was 271,500 pounds. Like N&W 76950, ACL 500000 was also a one-of-a-kind car; but it has fared much better than the N&W hopper and remained in service through the end of the century. After being sold for scrap, the car was purchased by the North Carolina Department of Transportaion and is on display at Rocky Mount.

In August 1965, aircraft builder Aeronca Manufacturing Co. of Middletown, Ohio built a 260-ton, 8-axle aluminum hopper car for the Southern Railway. This 103-foot car consisted of four permanently-connected 25-foot units, each riding on two axles. Southern 100 was loaded with inovations, including disk brakes, 7x12 bearings, and 38-inch wheels. The latter features gave the car a gross rail load of 630,000 pounds. It had a light weight of 96,000 pounds, a load limit of 534,000 pounds, and a volume of 9,000 cubic feet.

The Chesapeake & Ohio and Bethlehem Steel jointly developed a 120-ton, 6-axle hopper car. C&O 300011 consisted of two articulated parts and rode on three conventional 4-wheel trucks. Built in November 1968, the 56-foot car had a light weight of 65,200 pounds, a load limit of 243,300 pounds, and a volume of 4186 cubic feet.

The last noteworthy car from this era was Greenville's fully-enclosed Auto Guard tri-level automobile carrier. The 129-foot car could hold 18 full-size autos. The Auto Guard consisted of three permanently-connected 40-foot units, each riding on two axles. Only two were built, as Southern Railway 599000 and 599001, in September 1973. The cars had a light weight of 84,600 pounds and an indicated load limit of 100,000 pounds. They utilized truck components from hopper car 100, although these were refitted with 6½x12 bearings and 33-inch wheels. The 599001 was retired after an accident when it was only a couple of years old, but the other remained in service for 20 years. Norfolk Southern retired the 599000 in October 1993 and donated it to the Virginia Transportation Museum in Roanoke.

The one car type whose capacity most frequently exceeded 100 tons was the tank car. As the load limit increased, so did the number of axles; but the number of axles was tailored to a specific commodity and axle loading. Because of this, some 150-ton tank cars were built with six axles while others got eight. Approximately 681 of these RailWhales were built: 462 with six axles and 219 with eight axles.

The earliest RailWhales Union Tank Car (UTLX) 83699 built in March 1963 and General American (GATX) 96500 built in September 1965 had complex tank designs, but most RailWhales used a simple cylindrical tank. Both of these cars were engineering exercises to see just how many gallons one car could hold, but no other big cars of these designs were built. The UTLX car held 50,000 gallons and the GATX cars carried 60,000! Both of these cars are now in museums, the former at Galveston and the latter at St. Louis.

Almost all RailWhales were built between 1968 and 1970. There was quite a bit of variety in the size and gross weight of these cars, especially the 8-axle version. Cars designed to carry motor fuel anti-knock compound (tetraethyl lead) had a gross weight of 526,000 pounds, but held less than 28,000 gallons; while some of those designed for vinyl chloride service have the same gross weight but can hold over 48,000 gallons. Three of the biggest cars, CELX 7500-7502, held over 47,000 gallons; but had a gross weight of only 440,000 gallons. These cars hauled methanol.

The 6-axle cars ride on a pair of 6-wheel trucks, usually of the Buckeye design; while the 8-axle cars ride on two pairs of standard 4-wheel trucks mounted on span bolsters. Both 70- and 100-ton 4-wheel trucks were used. Although the 6-axle cars were more popular initially, some railroaders prefer the 8-axle cars. The longer rigid wheelbase of the 6-wheel trucks makes them more prone to derail on uneven track and sharp curves than the 4-wheel trucks on the 8-axle cars. Also, replacement parts for the 6-wheel trucks are much more expensive, and you probably won't find them in most car shops!

On 9 September 1970, the U.S. Departmant of Transportation issued a notice that the capacity of future tank cars should not exceed 34,500 gallons. This occurred after several accidents involving high-capacity tank cars. The big cars weren't more dangerous than smaller cars, but all of that cargo could be more of a hazard in an accident. This regulation (35 CFR 14216) effectively ended the construction of RailWhales after it went into effect on 15 November 1971. The regulation also ended the construction of 4-axle tank cars with 125-ton trucks.

The last large group of heavy-duty non-flat cars was seventy-five 150-ton bathtub gondolas built by Thrall for the Kansas City Southern in 1970. The 404000-series cars were built for coal service between Howe, Oklahoma and Port Arthur, Texas; but they have spent most of their lives hauling wood chips in Louisiana.

Today, half of the RailWhales have been retired; and because of the Association of American Railroads' 40-Year Rule, most of the remaining cars will probably be gone by 2010. If you see one of these impressive cars, you'd better photograph it because you may not get another chance!