Token (railway signalling)
In railway signalling, a token is a physical object which a train driver is required to have or see before entering onto a particular section of single track. The token is clearly endorsed with the names of the section to which it belongs. A token system is more commonly used for single lines because of the greater risk of collision in the event of a mistake being made by a signaller or traincrew than on double lines.
Principle
The operation of a bidirectional single track line has the hazard of two trains colliding. The simplest way to prevent such collisions is to have only one train in the section at any given time. Such a system is known as "one-engine-in-steam” or “one-train working". This system is used on some branches of rail networks, and on heritage railways. The main disadvantage is that it restricts the number of train movements that can be made. For a larger railway system, it becomes exceptionally limiting in the level of operations that it allows, with the increased risk of mistakes being made, possibly leading to a collision.Instead, reliance is placed not on employing only one train but on having a single physical object, known as a "token", available for the single track section, and ruling that a driver may enter the single line section only if in physical possession of that object. Existing in a variety of physical forms, such as a staff, tablet, key, or ball, the object is marked to indicate to which single track section it belongs.
Token systems
Token only
The token system was developed in Britain in the 19th century, to enable safe working of single-line railways. For the very first time this system was proposed by Henry Woodhouse for Standedge Tunnels in 1849. If a branch line is a dead end with a simple shuttle train service, then a single token is sufficient. The driver of any train entering the branch line must be in possession of the token, and no collision with another train is possible.For convenience in passing it from hand to hand, the token was often in the form of a staff, typically long and diameter, and is referred to as a train staff. Such a staff is usually literally a wooden staff with a brass plate stating the two signal boxes between which it is valid.
In UK terminology, this method of working on simple branch lines was originally referred to as One Engine in Steam , and later One-Train Working . However the system was used on long through lines as well; R H Dutton, Chairman of the London and South Western Railway explained in 1876 the slow journey time between Exeter and Plymouth by saying, "the cause of the delay is the stopping at every station on the staff system. That really does cause a great delay because if the staff is not there, the train must stop while a man is sent on a horse to get it "; quoted in Williams.
Staff and ticket
Using only a single token does not provide convenient operation when consecutive trains are to be worked in the same direction. The simple token system was therefore extended: if one train was to be followed by another in the same direction, the driver of the first train was required to be shown the token, but not take possession of it. The driver was given a written authority to enter the single line section, referred to as the ticket. The train could then proceed, and a second train could follow. In the earliest days, the second train could proceed after a designated time interval, as on double lines at the time. However, after the Armagh rail disaster of 1889, block working became mandatory.Seeing the train staff provided assurance that there could be no head-on collision. To ensure that the ticket was not issued incorrectly, a book of numbered tickets was kept in a locked box, the key to which was permanently fastened to the token, or was the token. In addition, the lock prevented the token from being removed until the ticket box was closed, and it could not be closed unless the book of tickets was in the box. Once a ticket was issued, its number was recorded in a Train Register book, and the token was locked in a secure place. The system is known as staff and ticket.
In a variation on that principle, called divisible train staff, a section of the token or staff, referred to as the ticket portion, was designed to be removed and handed to the driver, instead of a paper ticket.
Electric token
The staff and ticket system was still too inflexible for busy lines, as it did not allow for the situation where the train intended to carry the actual token was cancelled or running very late. To provide for this, the electric train token system was developed. Each single-line section is provided with a pair of token instruments, one at the signal box at each end. A supply of identical tokens is stored in the instruments, which are connected by telegraph lines. A token can be removed from one instrument only if both signalmen co-operate in agreeing to the release. Once a token has been removed, another cannot be removed until the token which is "out" is replaced in either instrument. By this means, it can be ensured that at any one time, only one token is available to be issued to a driver. Tokens belonging to adjacent sections have different configurations to prevent them being inserted into the wrong instrument.Nevertheless, a head-on collision occurred on a section of single track on the Cambrian Railways on 18 January 1918. The drivers of both engines held the correct token, issued from Tyer's token machines as they started their respective journeys, but the electrical circuits linking the machines at either end were also used for telephones and, together with a possible line fault caused by bad weather, this allowed the issue of two tokens at the same time. In the Abermule train collision of 1921, also on the Cambrian Railways, lax working procedures allowed the safeguards provided by the electric token system to be circumvented; a driver was handed a token for the wrong section, and proceeded on the mistaken belief that the token was correct. To try to prevent this, the UK Board of Trade Railway Inspectorate recommended that the signals controlling entry to the single line section were locked at danger unless a token had been released from the relevant token instrument. This was not universally adopted and many single line sections continued without such safeguards well into the 1960s.
Collection of the token
In a basic railway situation, the token can be collected personally by the driver at the start of the crew's work on a branch line, and surrendered at the end of their work there.Where the single line section is part of a through route, then it is likely that each passing train would require to surrender and collect a token at each token station. Where the trains stop at every station this is a convenient arrangement, but where some trains run through without requiring to make a call, it was necessary for the signalman to exchange tokens with the fireman as the train passed at slow speed. In the case of driver-only operated trains, a dead-mans hold over button was provided, so the driver could exchange the token without the emergency brake being applied.
A large staff could be handed over without any special apparatus, but if the system in use employed miniature staffs, tablets or key tokens, these were usually placed in a leather pouch attached to a hoop, and the fireman could put their arm through the hoop held up by the signalman, and vice versa as the locomotive ran past. In UK practice the permitted speed for this was in daylight, but there are stories of drivers anxious to make up lost time when running late, and passing the exchange point at much higher speeds; bruised upper arms were common among signalmen and firemen on such lines.
Fixed token exchange apparatus was used on some railways. Trackside equipment was fitted near each signal box to hold the pouch containing the token and to receive the token pouch that was being given up.
Automatic exchange
Certain railways developed mechanical systems that enabled faster handover using catcher devices. These could be extended from the locomotive cabside just before the train passed the exchange point and then automatically retracted afterwards. These enabled handover speeds of. Examples of such systems include the Whitaker system on the Somerset & Dorset Joint Railway and the Manson system on the Midland & Great Northern Joint Railway and the Great North of Scotland Railway. When the Aberdeen to Inverness passenger service was converted to diesel multiple unit operation in the late 1950s, the train-borne equipment was fixed adjacent to the train guard's compartment. A special buzzer code confirmed to the guard that the correct token had been successfully received.Mechanical staff exchangers were also used where trains did not stop on the single line sections of the Main South, Main North and North Coast lines of the New South Wales Government Railways in Australia. On the Victorian Railways automatic staff exchangers survived on the North Eastern, Western, and Geelong-Ballarat lines until as recently as the late 1980s. On the South Australian Railways, auto staff exchangers were used on the former broad-gauge line between Adelaide and Port Pirie. In all these Australian states, both steam and later diesel electric locomotives were fitted with auto exchangers. On the New Zealand Railways, where most lines are single track, multiple systems were used like the Winter’s block system and Sykes’ lock and block working. On the North Island Main Trunk and on other lines like the Wairarapa Line, the Tyer’s Electric Train Tablet with the No. 7 instrument was used. Tablet exchangers were developed by Wynne after trials of the Australian design by Quirke. The tablet system allowed for banking engines used for steep sections with a separate bank engine key.