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There are two common approaches to provide a continuous path for traction current that spans multiple track circuit blocks. The simplest method installs insulated track circuit joints on only one of the two rails with the second being a path for the return current and a ground for the track circuit rail.
In any given country, rail traffic generally runs to one side of a double-track line, not always the same side as road traffic. Thus in Belgium, China, France (apart from the classic lines of the former German Alsace and Lorraine), Sweden (apart from Malmö and further south), Switzerland, Italy and Portugal for example, the railways use left-hand running, while the roads use right-hand running.
The electrical bonding jumper connects the two rails to maintain continuity of the track circuit. Jointed track is made using lengths of rail, usually about 20 m (66 ft) long (in the UK) and 39 or 78 ft (12 or 24 m) long (in North America), bolted together using perforated steel plates known as fishplates (UK) or joint bars (North America).
Double junction, or level junction (left hand running). A double junction is a railway junction where a double-track railway splits into two double track lines. Usually, one line is the main line and carries traffic through the junction at normal speed, while the other track is a branch line that carries traffic through the junction at reduced speed.
This normally takes the place of an old absolute block section, and is commonly found where former absolute block sections and their associated signal boxes have been removed. Essentially an intermediate block section allows two block sections, and therefore two trains, to be on the same line but controlled by the same signal box. [9]
Generally, two separate two-wire circuits travel along the rail line between substations. One circuit is mounted at the top of the catenary poles on one side of the track; the second circuit runs along the other side.
A coded track circuit based system is essentially an inductive system that uses the running rails as information transmitter. The coded track circuits serve a dual purpose: to perform the train detection and rail continuity detection functions of a standard track circuit, and to continuously transmit signal indications to the train. The coded ...
With two signal protection, only one track circuit is needed between successive signals. However, overlaps may then be longer than is necessary with an adverse effect on headway. With one signal protection, there may be two track circuits between successive signals, which has a cost, however headways can be optimised.