It goes without saying that the object of this particular exercise is to prevent any possibility of a collision between trains following each other, or travelling in opposite directions, on a single line. At its most basic there is the 'One Engine in Steam' system. Only one engine (or MU, or engines coupled together) allowed on a section at any one time. In the very early days this was regulated by the use of a Pilotman who travelled on the engine but this was soon replaced by using a train Staff that the driver had to have in his possession at all times whilst in the section (unless some emergency occurred requiring assistance to be rendered to the train). This method had its drawbacks as trains had to run alternately, first in one direction, then in the other, as otherwise the Staff would be at the wrong end of the section. This basic method of operation was best suited to locations where the service always runs in alternate directions, in the main on dead end branches. The Southern Railway operated several lines using OES, for example between:

• Bentley and Bordon
• Brading and Bembridge
• Brookwood and Bisley Camp
• Brookwood and Necropolis
• Canterbury West and Whitstable Harbour
• Fort Brockhurst and Lee-on-Solent
• Hamworthy Junction and Hamworthy Goods
• Romney Junction and Dungeness

The driver would have in his possession the Staff, in the early days made of wood, later on of metal, on which would be engraved the names of either end of the section.

To get round the problem of the likelihood of the train Staff being at the wrong end of a section the system was developed first into the Staff and Ticket system, then into the Electric Tablet, Staff or Token system. The Staff and Ticket system was made possible with the introduction of the electric telegraph and block working. Now that quick and convenient communication could be made between the two ends of a section it became possible to send two or more consecutive trains through a section. The Staff for the section would be obtained but instead of being given to the first driver he would be shown the Staff and presented with a written authority (the 'Ticket') to proceed. Once confirmation was received from the other end that the first train had safely left the section the second driver (if three or more trains required to run) would be shown the Staff and and presented with a Ticket. This could, though highly unlikely, continue for many trains until the last train, the driver of which would carry the Staff through the section. This system, although a great improvement, was not foolproof as a misunderstanding could lead to a following train being allowed into the section before the preceding one had left it.

The next big step forward came in 1880 when the Caledonian Railway and Tyer & Co developed a system whereby a number of Tablets (in the form of discs) were held in two instruments, one at each end of a section, which were electrically interlocked so that only one might be withdrawn at any one time. This great leap forward meant that the Staff and Ticket system was no longer required as a Tablet could be withdrawn any time the section was not occupied, and for travel in whichever direction was required. To avoid confusion between adjacent sections, Tablets were made with holes of different shapes so that the Tablet for one section could not be put into the instrument for another and so that the driver could more easily identify the Tablet being passed to him. Various other systems followed, but all working to the same basic principle, which involved other forms of Tablet, Token or the Electric Train Staff, a metal staff in place of the disc of the Tablet or Token. Tokens and Staffs and were painted in different colours as well as having different configurations to prevent confusion betweem adjacent sections, though knowing the correct colour did not remove the obligation for him to check that the correct ends of section were engraved on the Staff or Tokent

Recent developments have involved the use of radio. Radio Electronic Token Block was developed primarily as a cost reduction exercise as it does away with the need for interlocking and frequent Signalboxes. The Token takes the form of an in-cab display which provides the authority to proceed, issued from a central computer.