Structure gauge in the context of "Gauntlet track"

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 70% 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.

A well car, also known as a double-stack car (or also intermodal car/container car), 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 that 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 — possible when bottom and top containers are the same dimensions, or through the use of inter-box connectors (IBC). Four IBCs are needed per well car. In the terminal there are four steps: unlock and lift off the top containers of an inbound train, remove the bottom containers, insert outbound bottom containers, lock assembly after top containers emplaced. Generally this is done car-by-car unless multiple crane apparatus are employed.

Engineering tolerance is the permissible limit or limits of variation in:

1. a physical dimension;
2. a measured value or physical property of a material, manufactured object, system, or service;
3. other measured values (such as temperature, humidity, etc.);
4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel (see structure gauge and loading gauge);
5. in mechanical engineering, the space between a bolt and a nut or a hole, etc.

Dimensions, properties, or conditions may have some variation without significantly affecting functioning of systems, machines, structures, etc. A variation beyond the tolerance (for example, a temperature that is too hot or too cold) is said to be noncompliant, rejected, or exceeding the tolerance.

A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge (distance between the rails) narrower than 1,435 mm (4 ft 8+1⁄2 in) standard gauge. Most narrow-gauge railways are between 600 mm (1 ft 11+5⁄8 in) and 1,067 mm (3 ft 6 in).

Since narrow-gauge railways are usually built with tighter curves, smaller structure gauges, and lighter rails, they can be less costly to build, equip, and operate than standard- or broad-gauge railways (particularly in mountainous or difficult terrain). Lower-cost narrow-gauge railways are often used in mountainous terrain, where engineering savings can be substantial. Lower-cost narrow-gauge railways are often built to serve industries as well as sparsely populated communities where the traffic potential would not justify the cost of a standard- or broad-gauge line. Narrow-gauge railways have specialised use in mines and other environments where a small structure gauge necessitates a small loading gauge.