On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable. Signalling control was originally exercised via a decentralised network of control points that were known by a variety of names including signal box (International and British) and interlocking tower (North America). London Underground call them signalling cabins, and the Great Central Railway referred to them as signal cabins. Currently these decentralised systems are being consolidated into wide scale signalling centres or dispatch offices. Whatever the form, signalling control provides an interface between the human signal operator and the lineside signalling equipment. The technical apparatus used to control switches (points), signals and block systems is called interlocking.

Originally, all signaling was done by mechanical means. Points and signals were operated locally from individual levers or handles, requiring the signalman to walk between the various pieces of equipment to set them in the required position for each train that passed. Before long, it was realized that control should be concentrated into one building, which came to be known as a signal box. The signal box provided a dry, climate-controlled space for the complex interlocking mechanics and also the signalman. The raised design of most signal boxes (which gave rise to the term "tower" in North America) also provided the signalman with a good view of the railway under his control. The first use of a signal box was by the London & Croydon Railway in 1843 to control the junction to Bricklayers Arms in London.

With the practical development of electric power, the complexity of a signal box was no longer limited by the distance a mechanical lever could work a set of points or a semaphore signal via a direct physical connection (or the space required by such connections). Power-operated switch points and signaling devices greatly expanded the territory that a single control point could operate from several hundred yards to several miles. As the technology of electric relay logic was developed, it no longer became necessary for signalmen to operate control devices with any sort of mechanical logic at all. With the jump to all electronic logic, physical presence was no longer needed and the individual control points could be consolidated to increase system efficiency.

Another advancement made possible by the replacement of mechanical control by all-electric systems was that the signalman's user interface could be enhanced to further improve productivity. The smaller size of electric toggles and push buttons put more functionality within reach of an individual signalman. Route-setting technology automated the setting of individual points and routes through busy junctions. Computerized video displays removed the physical interface altogether, replacing it with a point-and-click or touchscreen interface. Finally, the use of Automatic Route Setting removed the need for any human input at all as common train movements could be fully automated according to a schedule or other scripted logic.

Signal boxes also served as important communications hubs, connecting the disparate parts of a rail line and linking them together to allow the safe passage of trains. The first signaling systems were made possible by technology like the telegraph and block instrument that allowed adjacent signal boxes to communicate the status of a section of track. Later, the telephone put centralized dispatchers in contact with distant signal boxes, and radio even allowed direct communication with the trains themselves. The ultimate ability for data to be transmitted over long distances has proven the demise of most local control signal boxes. Signalmen next to the track are no longer needed to serve as the eyes and ears of the signaling system. Track circuits transmit train locations to distant control centers and data links allow direct manipulation of the points and signals.

While some railway systems have more signal boxes than others, most future signaling projects will result in increasing amounts of centralized control relegating the lineside signal box to niche or heritage applications.

In any node-based control system, proper identification is critical to ensuring that messages are properly received by their intended recipients. As such, signaling control points are provided with names or identifiers that minimize the likelihood of confusion during communications. Popular naming techniques include using nearby geographic references, line milepost numbers, sequence numbers, and identification codes. Geographic names can refer to a municipality or neighborhood, a nearby road or geographic feature, local landmarks, and industry that may provide the railway with traffic or railway features like yards, sidings, or junctions.

On systems where Morse code was in use it was common to assign control locations short identification codes to aid in efficient communication, although wherever signalling control locations are more numerous than mileposts, sequence numbers and codes are more likely to be employed. Entire rail systems or political areas may adopt a common naming convention. In Central Europe, for example, signalling control points were all issued regionally unique location codes based roughly on the point's location and function, while the American state of Texas sequentially numbered all interlockings for regulatory purposes.