Battery 50V—the positive side of 50V DC which is powered or supported by a battery back-up or UPS.

See also N50—the negative side of 50V DC.   

A back contact of a relay is made (makes an electrical circuit) when the relay is de-energised.

See also front contact.

The crushed rock (blue metal) foundation onto which the sleepers are fixed. The ballast allows excess water to drain away and locks together to hold the sleepers in place.

The track manager is typically responsible for everything "below" where the wheels of a train makes contact with the rail. "Below rail" therefore refers to the track infrastructure including the rails, stations, traction supply, signalling and communication systems, etc.

The rail operator is typically responsible for the rolling stock and the scheduling of services (above rail). 

See also above rail.

Referring to a section of track that has trains travelling in both directions.

In rural areas where the amount of rail traffic is lower than metropolitan areas, often there is only one track between two stations. A train may travel to the terminus and return along the same track (bi-directional). If multiple trains use the single track, the trains need to pass each other at stations or at passing loops. 

In metropolitan areas, it is common for two lines to be used. But two lines does not necessarily mean that one is used exclusively for the up-line traffic and one is used exclusively for the down line traffic. To permit timetable efficiencies, some tracks have been designed for bi-directional traffic during peak operations. This permits express trains for example to pass trains that are stopping at all stations.

A common track layout is three tracks. The two outer tracks may handle the normal train operations and the third track (or road) is used to speed other trains to their destinations. For example, this would be the outer suburbs to begin their passenger run in to town in the morning peak, or to speed the train to the main central station to begin their passenger run in the evening peak. The two outer tracks could still also be used for bi-directional traffic, adding to the complexity of options available.

Never assume that any track has trains travelling in one direction only!

Blocks are sections of track that are defined by their geographical features. A block may be between two stations, between a station and a turnout, between two turnouts, etc. Blocks are usually protected by signals to enter and to exit the block. This is called block signalling or in some cases, absolute blocks.

A signalling system where trains are kept separated by the signalling of each block.

In urban areas, blocks are short to reduce headway and thus increase the number of possible train movements for peak operation. Urban blocks typically rely on starter signals to leave a station, automatic signals between adjacent blocks, and home signals to enter a station. 

In rural areas, blocks are long (sometimes just one single block between stations or passing loops) as the traffic there is less dense. These blocks rely on a starter signal to enter the block and a home signal to enter the station (or passing loop).

Starter and home signals are absolute signals and directly controlled by the signaller. Automatic signals are permissive signals and typically operated via track circuits (track circuit signalling).

Block separation is much safer than time separation and much more costly to implement.

See also time separation.

Blocks are sections of track that are defined by their geographical features. A block may be between two stations, between a station and a turnout, between two turnouts, etc. Blocks are usually protected by signals to enter and to exit the block. This is called block signalling or in some cases, absolute blocks.

Whilst very common, block signalling has the diadvantage that all trains must adhere to one-size-fits-all rules and to worst-case-scenarios method of signalling. For example, block signals must be placed for the faster/heavier trains to allow for their large braking distances, even though some trains could slash this requirement.

The braking distance of a given train varies with certain conditions, such as:

• it increases as the mass or speed of the train increases.
• it increases where environmental conditions (rain, snow, volcanic ash, fallen leaves, etc.) reduce the friction between the rails and the wheels
• it increases on declines (and decreases on inclines) due to the effects of gravity
• it varies where different grades of steel are used in the rails vary the frictional qualities

A given train has a characteristic braking profile affected by its on-board features such as:

• disc brake or tread brake
• use of sand or gel to increase friction between the rails and wheels
• use of spin and slide detection software
• the frictional qualities of the wheel treads
• the type of braking system used (mechanical brakes, pneumatic brakes, electro-pneumatic brakes, dynamic brakes, etc)