Safeworking is the procedures and mechanisms for the safe operation of trains. This is an Australian term, but the principles, which includes signalling systems and token operations, are employed worldwide.
The goal of all safeworking systems is to prevent accidents on railway lines, particularly two trains running into each other.
The basic principle of all safeworking systems is to divide the railway lines up into "blocks", and to ensure that no more than one train is in each block at any one time. Blocks can be sections of line between stations, signal boxes (US: interlockings), signals, crossing loops, or other designated locations. Some modern computer-controlled systems are based on "moving blocks", in which the blocks are not between fixed locations, but move as the train moves. A block might then be defined as being a certain distance behind a moving train, and the computers ensure that no other train enters that block.
Some safeworking systems have to handle trains travelling over a section of line in the same direction, where the danger is that a following train catches up to a preceding train and runs into the back of it, especially if the previous train has stopped for some reason.
Other systems have to handle trains travelling over a section of line in both directions, as happens with single-track railway lines. This case has the additional danger of two trains entering the section from opposite ends and running head-on into each other.
Types of systems
Various safeworking systems have been developed, starting from very simple systems and progressing to more and more complex and foolproof systems. In many cases, these systems were developed or improved as the result of accidents. The result is that railways now are the safest form of transport.
One of the earliest safeworking systems introduced to railways was "timetable operation". In this system, trains are scheduled in such a way that no more than one train would be in any one section (block) at one time. This system only worked as long as the trains ran on time. If a train was delayed for any reason, another train might enter the same section, unaware that the previous train had not yet left the section.
Time interval working
An improvement on timetable operation was time interval working, in which railway police (as they were originally called in Britain) didn't allow a train to enter the next section of track until a given interval of time had passed since the previous train entered the section.
This had the effect of ensuring that two trains didn't enter the same section of track closely together, reducing the risk that the following train would catch up to the leading train and run into the back of it.
The railway police used flags by day and lamps by night to signal to the drivers of trains whether they were permitted to enter the next section of track or had to wait.
Block telegraph working
With the invention of the telegraph system, railways were able to improve safety by having the railway police, who became known as signalmen, control the entry of trains to sections based on whether or not the previous train had arrived at the other end of the section.
A system of bell codes was devised so that when a train arrived at the end of a section, the signalman could use the telegraph to send a message back to the signalman at the start of the section, advising him that the train had arrived and the section was now clear for the next train.
For single-line sections of railway, where the normal routine was for trains to pass through the section in alternate directions, a system known as token working, or staff working was developed.
In this system, each section of railway line has a "token", a "tablet", "key" or a "staff" dedicated to that section (usually with the name of the section engraved on it). Drivers were not permitted to drive their trains into these sections unless they had possession of the token. As there was only one token for each section, this ensured that only one train could be in each section at a time. The first train would carry the token from one end of the section to the other, where the driver would surrender the token to the signalman. The signalman would then give the token to the next train, which was travelling through the section in the other direction.
A problem for token working was that there were always times when two or more trains needed to travel through the section in one direction before one came back. If the first of these trains had the token, when the second train was ready to enter the section, the token was at the other end of the section, preventing the second train from going any further.
So a common variation on token working involves the use of a "ticket", normally a piece of paper on which is printed or written the name of the section and on which is written the date and number of the train. The signalman gives the ticket to the driver as his authorisation to be in the section, and, importantly, shows him the token. Seeing the token, the driver can be assured that there are no trains already in the section travelling in the opposite direction, as they would not have had the token to carry or sight.
When the train arrives at the end of the section, the driver surrenders the ticket to the signalman, who then telegraphs or telephones back to the signalman at the start of the section to say that the train has arrived. The first signalman is now free to send another train through the section, either by giving the driver of the next train the token, or again issuing the driver with a ticket and showing him the token.