Double Stack Containers and Unit Trains

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 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 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.

CSX lists three clearance heights above top of rail for double stack service:

• Doublestack 1 — 18 ft 2 in (5.54 m)
• Doublestack 2 — 19 ft 2 in (5.84 m)
• Doublestack 3 — 20 ft 2 in (6.15 m)

The last clearance offers the most flexibility, allowing two high cube containers to be stacked.

Dwarf Containers

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.

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 environmentally friendly electrical traction-based double-stack freight railway network which can transport international standard containers.

Unit Trains

A unit train, also called a block train or a trainload service, is a train in which all cars (wagons) carry the same commodity and are shipped from the same origin to the same destination, without being split up or stored en route. They are distinct from wagonload trains, which comprise differing numbers of cars for various customers.

Unit trains enable railways to compete more effectively with road and internal waterway transport systems. Time and money is saved by avoiding the complexities and delays that would otherwise be involved with assembling and disassembling trains at rail yards near the origin and destination. Unit trains are particularly efficient and economical for high-volume commodities. Since they often carry only one commodity, cars are of all the same type; often the cars are identical.

Unit trains are typically used for the transportation of bulk goods. These can be solid substances such as:

• Track ballast or gravel
• Iron ore from mines to ports or steel mills
• Coal from mines to power stations
• Coke from coking plants to steel mills
• Steel

Bulk liquids are transported in unit trains made up of tank cars, such as:

• Crude oil from oil fields to refineries (can be [60,000 barrels (9,500 m3)] of oil in a unit train of 100 tank cars)
• Mineral oil products from the refineries to the storage facilities
• Ethanol from ethanol plants to motor fuel blending facilities
• Molten sulfur (non-US:sulphur)

Food, such as:

• Wheat
• Corn
• Fruit juice
• Refrigerated food

Other examples include:

• Shipping containers, generally between a port and a truck depot
• Cars in autoracks
• Aggregate
• Military Equipment (weapons)
• Waste (garbage), usually for recycling, often metals or paper
• Potash
• Taconite
• Mail
• Sand for hydraulic fracturing