Double-stack rail transport is a form of intermodal freight transport where railroad cars carry two layers of intermodal containers. Introduced in North America in 1984, double stack has become increasingly common there, being used for nearly seventy percent of United States intermodal shipments. Using double stack technology, a freight train of a given length can carry roughly twice as many containers, sharply reducing transport costs per container. On most North American railroads, special well cars are used for double-stack shipment to reduce the needed vertical clearance and to lower the center of gravity of a loaded car. In addition, the well car design reduces damage in transit and provides greater cargo security by cradling the lower containers so their doors cannot be opened. A succession of larger container sizes have been introduced to further increase shipping productivity on shipments within North America.
Double-stack rail operations are growing in other parts of the world, but are often constrained by clearance and other infrastructure limitations.
Double-stack cars come in a number of sizes, related to the standard sizes of the containers they are designed to carry. Well lengths of 12.19 m (40.0 ft), 14.63 m (48.0 ft) and 16.15 m (53.0 ft) are most common. Heights range from 2.44 m (8 ft 0 in) to 2.90 m (9 ft 6 in)("high cube").
Double stack requires a higher clearance above the tracks, or structure gauge, than do other forms of rail freight. Double-stack cars are most common in North America where intermodal traffic is heavy and electrification is less widespread; thus overhead clearances are typically more manageable. Nonetheless, North American railroads have invested large sums to raise bridges and tunnel clearances along their routes and remove other obstacles to allow greater use of double stack trains and to give them more direct routes.
The last clearance offers the most flexibility, allowing two high cube containers to be stacked.
Intermodal containers shipped by rail within North America are primarily 53 feet (16.15 m) long, with trailer-on-flat-car (TOFC) units used as well. The 53-foot length reflects a common maximum length for highway semi-trailers, which varies by state. Major domestic intermodal carriers include:
Containers shipped between North America and other continents consist of mostly 40-foot (12.19 m) and some 45-foot (13.72 m) and 20-foot (6.10 m) containers. Container ships only take 40's, 20's and also 45's above deck. 90% of the containers that these ships carry are 40-footers and 90% of the world's freight moves on container ships; so 81% of the world's freight moves by 40-foot containers. Most of these 40-foot containers are owned by non-U.S. companies like Maersk, MSC, and CMA CGM. The only U.S. 40-foot container companies are leasing companies like Textainer, Triton International, and CAI Leasing. .
Southern Pacific Railroad (SP), along with Malcom McLean, devised the double-stack intermodal car in 1977. SP then designed the first car with ACF Industries that same year.[page needed] At first it was slow to become an industry standard, then in 1984 American President Lines, started working with the Union Pacific Railroad and that same year, the first all double-stack train left Los Angeles, California for South Kearny, New Jersey, under the name of "Stacktrain" rail service. Along the way the train transferred from the UP to the Chicago and North Western Railway and then to Conrail.
Low bridges and narrow tunnels in various locations prevent the operation of double-stack trains until costly upgrades are made. Some Class I railroad companies in the U.S., often in partnership with government agencies, have implemented improvement programs to remove obstructions to double-stack trains. Double-stack projects include:
Forty-foot containers are the standard unit length and load bearing points are at the ends of such containers. Longer containers, such as 45, 48 and 53 feet long, still have the load bearing points 40 feet apart, with the excess protruding equally outside this length. Therefore, 40-foot containers, or larger, can be stacked on 20-foot containers if there are two 20-foot containers in a row; however, 20-foot containers cannot be stacked on top of 40-foot or longer containers. The possible double-stacking patterns are:
The container coupling holes are all female and double male twistlocks are required to securely mate container stacks together.
China had started to use reduced size containers to be stacked onto normal containers to allow transport under 25 kV electrification. It did not allow for combination with hi-cube containers though.
India has started to build a series of dwarf container for domestic transport to be run under 25 kV electrification. With 6 feet 4 inches (1,930 mm) they are 662 mm shorter but 162 mm wider than ISO shipping containers while still allowing for 67% more capacity. The chosen width is comparable to the American 53-foot containers.
Containers have weight limits designed to allow their transport by road trucks, which have lower weight limits than trains. Outside North America, a common limit for railways is 8 tonnes per metre (8.1 short ton/yd; 7.2 long ton/yd) train length and 22.5 tonnes (22.1 long tons; 24.8 short tons) per axle. A four axle container car can take 90 tonnes (99.2 short tons; 88.6 long tons). Since a container is limited to 30.5 tonnes (33.6 short tons; 30.0 long tons), even including the empty weight of the rail car, single stacking uses only part of the load capacity of the railway. A 20-foot (6.1 m) container is limited to 24 tonnes (26.5 short tons; 23.6 long tons) and two such can fit into a car for a 40-foot (12.2 m) container, or even three if double-stacking, but not four unless very high axle load is permitted[why?]. The North American railways permit two 53-foot (16.15 m) or four 20-foot (6.1 m) containers as shown in the images on this page.
Another consideration is the maximum weight of a train. A maximum length train in Europe, 750 m (2,461 ft) long can have 50 container cars with a total weight of 2,250 tonnes (2,480 short tons; 2,210 long tons), and more if 20 ft containers are included. This is not far from the limit using standard European couplers. Double-stacking requires allowing higher train weight to be meaningful, since it is higher train weights that saves costs. In the U.S., the AAR coupler used allows much higher train weight. In any case, the European loading and structure gauges do not allow double stacking.
On the vast majority of its network, Europe has more restricted loading gauge and train weight limits as well as axle load limits. In other words, many bridges and tunnels are too low for double-stacking. In addition, since European electrification standards generally predated double stacking overhead catenary in Europe is too low to accommodate double stacking and rebuilding for double-stack operation is far too expensive. Only a few newly built routes make accommodation for possible double stacking in the future such as the Betuweroute in the Netherlands which however links to no other railway line allowing double stacking.
Standard gauge railways in North America and China must use special well wagons to lower the center of gravity, reduce the loading gauge and in China allow double stack trains to run under specially heightened overhead lines. 1,676 mm (5 ft 6 in) broad gauge used in India and Pakistan allows for trains can carry standard shipping containers double-stacked on standard flat wagons. Flat wagons, in addition to being much less expensive than well wagons, can carry more containers in a given length of train. Indian Railways is able to carry containers double-stacked on standard flat wagons at 100 km/h (62 mph). Experiments with triple-stacked operation using lower, 1,981 mm (6.5 ft) containers, were done unsuccessfully in 2006. Experiments in India for double stacking using flatcars under 25 kV AC overhead lines set 7.45 m (24.4 ft) above rail have begun with funds given by Japan.