Container chassis

A container chassis, also called intermodal chassis or skeletal trailer, is a type of semi-trailer designed to transport shipping containers. It is a wheeled metal frame onto which a container can be mounted for hauling over roads by a semi-tractor. Container chassis are an important component of intermodal freight transport.
Intermodal.
Chassis are used by truckers to transport shipping containers between ports, railyards, container depots, and shipper facilities. This type of trucking is referred to as drayage or intermodal trucking. Drayage is usually the very last or first part of a container's intermodal journey. Intermodal is the use of different transportation modes to deliver cargo. 
Unlike containers, chassis are not transported overseas. They are only used to haul containers within a port or from a port to a shipper and vice versa. 
Operation.
A crane is used to place a shipping container on a chassis, ensuring that the container's corner castings line up with the chassis’ twistlocks (pins). The container is then locked to the chassis by engaging the pins. To lift the container off the chassis, the pins are first disengaged. Lifting a container on or off a chassis is referred to as a flip. A semi-tractor can hook up to the chassis to pull it and deliver the shipping container. When not hooked up to a tractor, container chassis have a pair of legs called landing gear which can be lowered to support the chassis. The landing gear are cranked up when the chassis is hooked up to a truck.
Like any other semi-trailer, chassis have a kingpin which hooks up to the tractor's fifth wheel. Truck drivers also connect the air and electrical lines from their truck to the chassis' gladhand sockets. The air and electricity lines control the emergency brake and lights on the chassis, respectively.
Portable electric generators, also called gensets, can be mounted (underslung) onto chassis. The gensets are used to power a refrigerated container (reefer).
In the US, some chassis, especially 20-foot and 53-foot chassis, have sliding tandems. The tandems are pulled back for heavy containers to comply with federal bridge law weight restrictions.
An identification number is usually stenciled on chassis to keep track of each unit in a fleet. According to ISO 6346, a chassis should have the letter "Z" at the end of its reporting mark. For example, ABCZ-123456 7 would mean the equipment is a chassis, specifically, number 1234567 in the fleet of company ABC. 
Types.
The length of a chassis determines which container length it can accept. The standard ISO containers are either 20-, 40-, or 45-foot-long. For example, a 40-foot-long chassis is used for a 40-foot-long container. While some chassis are fixed length, others can be extended or shortened depending on which container needs to be hauled. The most common adjustable chassis lengths are 40/45. 
In some countries there are other lengths of containers used domestically. For example, in North America there are 53-foot-long containers used only within North America. For these 53-foot-long containers, there are 53-foot-long chassis.
Logistics.
United States.
In the US, there are two ways container terminals operate. The first is called a "grounded" or "stacked" terminal. At these terminals, containers are stacked on the ground. When drivers need to pick up a box from a grounded terminal, they must bring a bare chassis with them or find one somewhere in the terminal. Unless the driver has a privately-owned chassis, then they need to get a chassis from a chassis pool. Chassis pools are stored at container terminals and are owned by leasing companies. The pool chassis were formerly owned by the steamship lines and lent to truckers for free, but the steamship lines sold them to the leasing companies in the late 2000s. Once the driver has a bare chassis, they take it to where their container is stacked, and the crane operator lifts the container onto their chassis.
The other type of terminal is called "wheeled"—terminal workers place containers individually on pool chassis and park them before truckers arrive. When truckers arrive, they hook up to the chassis holding their container and drive away. If drivers wish to use a different chassis, they need to get a flip.
Whenever a driver out-gates or in-gates a pool chassis from a terminal, the terminal sends an EDI containing the chassis number, container number (if there is a container), trucking company, and other details to the chassis pool company. The chassis pool company uses this information to invoice the trucking company for daily usage of the chassis. Then, the trucking company passes this cost to their customer. 
There are two important caveats in this process. First, the steamship line tells wheeled terminals which chassis pool to use with its boxes. Second, in cases when the steamship line is buying the drayage service (called carrier haulage), the steamship line tells the trucker which chassis pool to use. These caveats are because the steamship lines signed contracts with the chassis pools when they divested from chassis. In these contracts, the steamship line negotiated a rental rate with a particular pool. Trucking companies and container terminals have a list of which steamship lines go with which chassis pool. Technically, when picking up a carrier haulage box from grounded terminals, drivers should bring a bare chassis from the steamship line’s chosen pool. Pool chassis are stenciled with the name of pool they belong to, usually a 4-letter code like "COCP". If the pool is unavailable at the terminal where the box is located, the driver must first bobtail to another terminal to pick up a chassis (called chassis split). Similarly, at wheeled terminals, drivers need to check if the terminal placed the box on the correct chassis pool before they out-gate. In carrier haulage situations, if the trucker does not use the steamship line’s chosen chassis pool, the steamship line usually will not pay any chassis costs. However, if the shipper/forwarder are buying the drayage service (called merchant haulage), the trucker and shipper can make their own chassis arrangements (at wheeled terminals, they would need to get a flip to move the container from the provided chassis onto their desired chassis). 
Other countries.
In most countries other than the US, truckers own or long-term lease container chassis—steamship lines have no influence on chassis. 
Shortages.
In the United States, container chassis shortages are a chronic problem, especially during peaks in freight volume. There are several causes of chassis shortages, but a common problem is excessive off-terminal dwell time. Off-terminal dwell time is the length of time a shipper keeps a chassis/container at their premises. Long dwell times cause shortages at ports and rail ramps where incoming containers need to be loaded onto chassis. 