Double-stack rail transport
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.
Sizes and clearancesEdit
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.
- Doublestack 1 — 5.54 m (18 ft 2 1⁄8 in)
- Doublestack 2 — 5.84 m (19 ft 1 7⁄8 in)
- Doublestack 3 — 6.15 m (20 ft 2 1⁄8 in)
The last clearance offers the most flexibility, allowing two high cube containers to be stacked.
International and domestic stack trains in North AmericaEdit
Intermodal containers shipped by rail within in 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:
- JB Hunt
- Schneider National
- Hub Group
Containers shipped between North America and other continents consist of mostly 40 feet (12.19 m) and some 45 feet (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 80% 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.
Double stack projects built or proposed in North AmericaEdit
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:
- Heartland Corridor (Norfolk Southern Railway) — $320 million
- Norfolk Southern Crescent Corridor — $2.5 billion (estimate)
- National Gateway (CSX Transportation) — $700 million 
- Commonwealth Railway — $69 million
- Chicago Region Environmental and Transportation Efficiency Program — $3 billion (estimate)
- Cross-Harbor Rail Tunnel proposed to connect rail freight lines in New Jersey with Long Island, New York. — $10–14 billion (estimate)
- Continental Rail Gateway (Canadian Pacific Railway), proposed tunnel between Detroit, Michigan and Windsor, Ontario to replace the existing Michigan Central Railway Tunnel. — $400 million
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:
- Two 20 ft in lower and one 40 ft (or longer) in upper stack (allowed in India, China)
- One 40 ft in lower and another 40 ft (or longer) in upper stack (allowed in India)
- Two 20 ft in lower and another set of two 20 ft in upper stack (possible in well-type cars, where ?)
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.
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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. 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.
Outside North AmericaEdit
Since electrification generally predated double stacking in Europe and New Zealand, their overhead catenary is too low to accommodate it. Many bridges and tunnels are too low for double-stacking, rebuilding for double-stack is far too expensive. However, India is building some freight-only corridors with the overhead wiring at 7.45 m (24.4 ft) above rail, which is high enough.
Because of the 1,676 mm (5 ft 6 in) broad gauge used in India and Pakistan, trains can carry standard shipping containers double-stacked on standard flat wagons, which is more economical than single containers. Standard gauge railways in North America and elsewhere must use special double-stack cars or well wagons to lower the center of gravity and reduce the loading gauge. Indian Railways is able to carry containers double-stacked on standard flat wagons at 100 km/h (62 mph). (Triple-stacked operation with lower, 1,981 mm (6.5 ft) containers, was mooted without success in 2006). Flatbeds, in addition to being much less expensive than well wagons, can carry more containers in a given length of train. Experiments for double stacking under 25 kV AC overhead lines have begun because of funds given by Japan.
- Australia - double-stacked trains operate between Perth, Adelaide, Darwin and Parkes, New South Wales 6.5 m (21 ft) clearances . The Inland Railway between Melbourne and Brisbane will be built for the operation of double stacked trains.
- Brazil - Brazil is one more country to be using double-stacked container trains soon. The project is from Brado Logística, and will be on the route between the intermodal terminal of Sumaré-SP to Rondonópolis-MT (around 1000km). In 2019, the first lot of double-stack car has been available for use, but with one container for each wagon (single-stack). Some works on the railroad such as some bridges, is on the way to finish up, but with no specifc date.
- China - using double-stacked container trains under 25 kV AC overhead lines.
- India - Mundra Port and Pipavav Port operate double-stacked diesel trains on 1,676 mm (5 ft 6 in) gauge using flat wagons. It is one of only three countries[which?] to commercially double stack 9 ft 6 in (2,896 mm) tall (high cube) containers on a train. India is building the Dedicated Freight Corridor, an economical and environmental friendly electrical traction based double-stack freight railway network which can transport international standard containers, a first in the world.
- Kenya - The Mombasa-Nairobi standard gauge railway has been built for the operation of double-stacked trains, the first such trains being launched on October 1st 2018. All future extensions will be built to similar standards.
- Netherlands - The Betuweroute, a freight rail between Rotterdam to the German border built 2007 is prepared for double-stacking insofar that tunnels are being built to accommodate double stack trains to reduce the cost and time of any future upgrade to the network. The current catenary wire is too low for double stack rail transport and the upgrade is dependent on the German section of the railway connection.
- Panama - The rebuilt Panama Canal Railway operates double-stack container trains 47.6 miles (76.6 km) across the Isthmus of Panama from Colón on the Atlantic Ocean, to Balboa on the Pacific, near Panama City.
- Saudi Arabia - Saudi Railways Organization line to Dammam.
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