Double-stack rail transport
Double-stack rail transport is a form of intermodal freight transport in which railroad cars carry two layers of intermodal containers. Invented in the United States in 1984 it is now 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 United States 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 in the United States. Double-stack rail operations are growing in other parts of the world, but are often constrained by clearance and other infrastructure limitations. History. 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. 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. Sizes and clearances. 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 , and are most common. Heights range from  to ("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. Outside North America some rail routes have been built or upgraded to such standards as to  allow both electrification with overhead wires and double stacking. CSX lists three clearance heights above top of rail for double stack service: The last 2 clearances offer the most flexibility, allowing two high cube containers to be stacked. Stacking containers. 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. Dwarf containers. China had started to experiment using reduced-size containers to be stacked on to normal containers to allow transport under 25 kV overhead line electrification. India has started to build a series of "dwarf container" for domestic transport to be run under 25 kV electrification. With a height  of they are 662 mm shorter than ISO shipping containers but 162 mm wider while still allowing for 67% more capacity when double stacked as compared to single stack ISO container. The width is comparable to that of North American 53-foot containers. Weights. Containers have weight limits designed to allow their transport by road trucks, which have lower weight limits than trains. Outside the U.S., a common limit for railways is train length and per axle. A four axle container car can take . Since a container is limited to , even including the empty weight of the rail car, single  stacking uses only part of the load capacity of the railway. A container is limited to and two such can fit into a car for a container, or even three if double-stacking, but not four unless very high axle load is permitted. The North American railways permit two or four containers as shown in the images on this page. Another consideration is the maximum weight of a train. A maximum length train in Europe, long can have 50 container cars with a total weight of , and more if 20 ft containers are included. This is not far from the  limit using standard European (freight) couplers. Operations. North America. Intermodal containers shipped by rail within North America are primarily 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 and some and 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. 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: Outside  North America. 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 cars to lower the center of gravity, reduce the loading gauge and in China allow double stack trains to run under specially heightened overhead lines. broad gauge used in India enables trains to 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 . Experiments with triple-stacked operation using lower , containers, were done unsuccessfully in 2006. Experiments in India for double stacking using flatcars under 25 kV AC overhead lines set above rail have begun with funds given by Japan.