A well car, also known as a double-stack car or stack car (also well wagon), is a type of railroad car specially designed to carry intermodal containers (shipping containers) used in intermodal freight transport. The "well" is a depressed section which sits close to the rails between the wheel trucks of the car, allowing a container to be carried lower than on a traditional flatcar. This makes it possible to carry a stack of two containers per unit on railway lines (double-stack rail transport) wherever the structure gauge assures sufficient clearance. The top container is secured to the bottom container either by a bulkhead built into the car (e.g., bottom and top containers are the same dimensions of 40 ft.), or through the use of inter-box connectors (IBC). Four IBCs are needed per wellcar. In the process of an inbound train becoming an outbound train, there are four processes: unlock to unload the top container of inbound train, remove then unload bottom container, insert after loading bottom container of outbound train, lock after top container loaded.
Double-stack cars are most common in North America and Australia where intermodal traffic is heavy and electrification is less widespread; thus overhead clearances are typically more manageable. In India double stacking of containers is done on flatcars under 7.45 m (24 ft 5+1⁄4 in) high catenary because the wider 5 ft 6 in (1,676 mm) gauge permits more height while keeping the centre of gravity still low.
Southern Pacific Railroad (SP), along with Malcom McLean, devised the first 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 Thrall Company to develop a refined well car and with the Union Pacific to operate a train service using the new well cars. That same year, the first all "double stack" train left Los Angeles for South Kearny, New Jersey, under the name of "Stacktrain" rail service. Along the way the train transferred from the UP to CNW and then to Conrail.
Each unit of a double-stack car is constructed with a single well, but are often constructed with multiple units of three to five units, connected by articulated connectors. Articulated connectors are supported by the centerplate of a single truck, (often a 125-short-ton, 112-long-ton or 113-tonne capacity truck but sometimes a 150-short-ton, 134-long-ton or 136-tonne capacity one).
On both types of multiple-unit cars, the units are typically distinguished by letters, with the unit on one end being the "A" unit, and the unit on the other end being the "B" unit. Middle units are labeled starting with "C", and going up to "E" for five-unit cars starting from the "B" unit and increasing towards the "A" unit.
Double-stack wellcars come in a number of sizes, related to the standard sizes of the containers they are designed to carry. Well lengths of 40 ft (12.19 m), 48 ft (14.63 m) and 53 ft (16.15 m) are most common. A number of 45 ft (13.72 m) wells and 56 ft (17.07 m) wells also exist. (The sizes of wells are frequently marked in large letters on the sides of cars to assist yard workers in locating suitable equipment for freight loads.)
On 40 ft, 45 ft, 48 ft, or 53 ft cars, larger containers (45 ft or up) are often placed on top of smaller containers that fit in the available wells to efficiently use all available space. All wells are also capable of carrying two 20 ft ISO containers in the bottom position.
Articulated well cars typically have a capacity of 120,000 lb (54,000 kg) per well. As highway weight limits in the US restrict most containers to less than 60,000 lb (27,000 kg) this is adequate for two containers stacked. Some single well cars have capacity for two fully loaded 32,500 kg (71,700 lb) containers.
Econo Stack well cars are a variation of conventional well cars and their main purpose is to give the double stacked containers more support. The down side to them is they do not allow 53-foot containers to be stacked on top, but 45-foot containers still fit and are able to be stacked on top.
A train of well cars in Arizona
A well car loaded with two forty foot containers of Transportación marítima mexicana (TMM).
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. have initiated improvement programs to remove obstructions to double-stack trains. Examples include the Heartland Corridor (Norfolk Southern Railway) and National Gateway (CSX Transportation).
Increasing the height clearance for trains to 6500 mm between Parkes and Crystal Brook will allow a larger range of double-stacked container combinations to be carried