The locking and securing of freight containers, equipped with the hollow International Standard Organization (ISO) bottom corner castings or their equivalent, to load carriers such as a truck chassis or rail car, has been pursued in different ways for more than 30 years.
Over the years, different types securing devices for maintaining freight containers on load carriers have been developed. Depending on the circumstances of the security required, three main categories of locks have been developed:
A. Positioning locks which do not provide vertical hold down of the container on the carrier. Such locks are inexpensive in both manufacture and usage. They are commonly used on rail cars in Europe. Until recently, the disadvantage of not having vertical hold down on the positioning locks used on rail cars has been acceptable because such locks do not require labor for locking or unlocking. However, European railroads are reconsidering use of such locks because empty freight containers without vertical hold down have been blown off carrier cars when passing over high bridges during exposure to sidewinds.
B. Positive locking securing devices, which provide vertical hold down of the container on the load carrier up to a certain force. This type of lock permits, the lock to disengage if a predetermined lifting force is exceeded. This type of lock is commonly used on rail cars in the USA and Canada as it is inexpensive to operate and provides reasonable securing safety.
C. Positive locking securing devices which provide vertical hold down of the container on the carrier unless unlocked or broken. Such locks are commonly designed as a stem with a rectangular upward pointed head. After loading the container over the pointed head, the container is locked to the carrier by manually twisting the stem about 90 degrees, hence the name twistlock. Twistlocks are normally used on a truck chassis in three different designs: non-retractable, retractable (below the horizontal load line) and screw down (the head and the stem can be tightened down to eliminate play between head and corner casting during transportation).
A large variety of such twistlocks have been designed. This type of lock is more time consuming to handle since they all have to be individually and manually operated. The obvious advantage is however that the lock offers a high level of locking safety as it will only release if it is pulled to a break point. Further since they normally are used on a truck chassis rather than a rail car, the truck driver is always available to operate the locks on his truck, so manually operated twistlocks that offer positive locking, do not present costly labor costs.
Notwithstanding, the availability of the above identified locking devices, operating positively locking twistlocks on rail cars has been proven to be very labor intensive. For instance British Railways has fitted 24 regular truck chassis twistlocks to its rail cars to better secure cargo containers. However, the amount of time required to unfasten these twistlocks is viewed as a major disadvantage.
Because of the time consumed in removing a cargo container from the rail cars in the type of system used by the British, other railroads have been looking for a less complicated system. In Europe, the need for a less time consuming securing system with positive locking is driven not only by adverse experience with position locks but by the number of different types of containers in use. These include the standard ISO 20', 30' and 40' containers as well as European Swap Bodies, with locking points at 20' but gross length of the containers being, for example, 23.6 feet, or 25.7 feet long. The number of different length containers greatly increases the number of different load configurations that the securing system on the load carrier must be capable of handling. For example--a 60' rail car must have 22 pairs of twistlocks in pre-set positions, in order to accommodate all possible load patterns. The locks also must be retractable, so that the right number of locks in the right positions to suit a certain load configuration, are exposed. It is also desirable to permit longitudinal movement of the locks along the rail car to reduce the number of locks needed.