When the driver learns how to interpret the scale properly through experience, combined with their knowledge of how much the rig weighs when empty, they can interpret how much cargo can safely and legally be loaded onto the trailer. With different varying loads of cargo, the driver can have an idea how much the gross total weight is, and if they are legal to avoid a ticket (80,000 pounds without a permit in most states, but slightly lower in others). Some decks have pop-up chain systems which have a higher WLL (working load limit) than attaching chains to either the stake pocket/spools, or the frame. Other decks of trailers can have sliding detachable accessories which greatly diversify the options of where and how to place a chain hook for securement. Besides axles which raise/lower as needed, some spread axle trailers can slide one or both axles forward, or back to create a tandem setup in specific situations when necessary, to comply with weight distribution requirements. Certain amounts of front and rear cargo overhang are allowed (as well as overhang to one or both sides of the trailer) with flags/banners/flashing lights to warn drivers behind and to the side of impending danger of impalement if they follow too closely behind and the truck suddenly stops. In extreme cases, permit loads require an escort vehicle in the front, rear, or both for oversize/over dimension cargo/equipment. Tow trucks Some vehicle recovery tow trucks have flat beds and are able to winch a recovered vehicle entirely on board. They can then drive the vehicle away for repair without needing to tow it. This allows a faster journey, does not require a driver in the vehicle being towed, and allows a damaged vehicle to be recovered when it cannot be towed. As these flat beds usually slope gradually to the rear, unlike the level bed of a cargo flatbed, they are known as 'beavertails'. Some tow truck beds are demountable and may be lowered behind the truck for easy loading, then both bed and load winched back aboard as one. Railway flatbeds Railways also employ flatbed trucks in engineering trains and freight trains. In Britain and the Commonwealth the term bogie flat is often applied to a bogie flatbed truck. Although less common, flatbed railway trucks on rigid frames and axles are sometimes used, with both 4-wheel and 6-wheel versions being extant. In British English,