Signalling, command and train control

The importance of signalling and controlling train movements was recognised from the early days of railways. In principal, such systems aim at providing train drivers sufficient time to stop trains before meeting any obstacles ahead, with minimal reduction of line usage (De Fontgalland, 1984). Due to the individual train characteristics, e.g. length, weight and adhesion, braking cannot be immediate. Thus signalling is essential for informing drivers well in advance of the actions that they should take, e.g. reducing train speed or proceeding at normal speed. According to Bonnet (2005) although modern signalling systems may differ not only between but also within countries, they should all have the following objectives: (i) control trains in a safe manner for the conditions ahead; (ii) maintain a safe distance to any train in front or behind; (iii) prevent the setting of conflicting movements; (iv) ensure that switches are locked in the correct position; (v) enable trains to operate at the scheduled speed with a minimum of possible disruptions, without affecting the safety of operations; and (vi) assure that trains always maintain the minimum required time interval between them. The operational concept of the modern railway signalling is developed on the “block system” concept (Pasquini et al., 2004). A route line is divided into blocks, where each is protected by a signal. The operational principle can be summed up to “no more than one train shall be in a block section on the same line at the same time” (Hall, 2005). If a Block n is occupied then the signal protecting that block should be set to red, while the preceding signal should

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be yellow, as illustrated in Figure 2-4. The yellow signal alerts the driver that the signal ahead is likely to be red, unless the block is cleared during the time required for the train to move between the two signals. For normal running movements different types of signals are used, including, amongst others, the “two”, “three” and “four” aspect colour signals. For other specific movements, i.e. shunting operations, small signals are used (ground or dwarf signals). Signals can be found at the start of a block, on the approach to switches, before other signals and level crossings, at stations or ahead of junctions. To ensure that trains have sufficient time to stop, the distance between signals depends on the line speed, gradient of the track and a safety margin specified by the designers (De Fontgalland, 1984, Bonnett, 2005).

Figure 2-4 Signalling blocks protection (Pasquini et al., 2004)

In addition to signalling, there are many protection systems implemented worldwide, which all primarily aim to warn train drivers of signals and their indications ahead. In addition, many of them are able to stop trains automatically when drivers do not obey signals indications. Some of the best-known protection systems, as described by Hall (2005), are :

• The Automatic Warning System (AWS). This is widely used in the UK and Australia primarily on main line operations for both passengers and freight trains. AWS is designed to warn train drivers of the need to slow down or stop their trains.

• The Train Protection and Warning System (TPWS). Widely used in the UK and Australia, TPWS has been developed as an improvement of the AWS. It is used to reduce the consequences of Signals Passed at Danger (SPADs). The TPWS is designed to capture the probability of a driver acknowledging the AWS warning indicators but failing to apply braking appropriately. Subsequently, if the train passes the signal at danger, the TPWS will force the train to stop regardless of the actions of the train driver.

• The Automatic Train Protection (ATP). ATP is widely applied in the US, UK and

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limits. ATP differs from AWS and TPWS, as it applies braking automatically if it finds that train drivers fail to reduce train speed to the extent required. ATP is very popular on metro services due to the very dense train services.

• The Integra-Signum protection system. This is used in Switzerland and is similar to the TPWS in stopping the train automatically when train drivers either do not acknowledge signals ahead or pass signals at danger.

• The European Train Control System (ETCS). ECTS has been developed to introduce a single train protection system on the European network in order to simplify cross- country and within countries operations. There are three ECTS levels, Level 1, 2 and 3. Depending on the Level, train controls rely on: the conventional line side signals and ATP system (Level 1); the conventional detection systems and information that drivers receive by a secure radio system, referred to as the Global System for Mobile communication-Railway GSM-R (Level 2); on-board calculations of train location and transmission to control centres by GSM-R (Level 3).

With respect to the concept of controlling the train movements, this lies on the “track circuit” idea, which is considered as the heart of the modern signalling (De Fontgalland, 1984, Hall, 2005). It is the simplest and probably most efficient method to identify if a train is operating on a particular length of a track and to set up the signals and switches accordingly (Bonnett, 2005). The passage of the wheels over a section activates an electrical circuit between the rails which subsequently turns on the relays and causes the signal and switches to change (De Fontgalland, 1984, Hall, 2005). The track circuit is responsible for a number of essential functions, such as: holding signals in the rear at danger; locking the facing points in the route direction; notifying signallers of the presence of a train; changing signals ahead from danger to proceed aspect; supporting signallers to keep in step with train movements; and finally giving warnings of broken rails or obstacles on the line (Hall, 2005). Finally, in addition to track circuit, the interlocking of signals and switches is another way to enhance safety of railway operations. Interlocking has been used to prevent signallers accidentally clearing a signal before the points are properly set and to clear signals that could lead to a conflicting movement (Bonnett, 2005).

In addition to the signalling and control of trains, the railway communication systems (RCS) are vital for a safe and secure railway operation. Similar to the railway protection systems, the RCS may differ across countries. However, they all aim to provide and maintain secure communication between trains and control rooms along the rail network, as well as provide passengers with essential journey information. Radio is the primary means of communication, while in some cases the use of telephones is permitted (Bonnett, 2005). In

most European countries and others including India, China and Saudi Arabia, national telecommunication railway systems, e.g. the Cab Secure Radio (CSR) and the National Radio Network (NRN) in the UK, are gradually being replaced by GSM-R (Smith et al., 2013). This improves safety and facilitates the interoperability of the railway operations.

2.2.2.1.3 Electrification

Electrification provides trains with electrical energy, which is transferred either by overhead or earthing equipment. The scheme is characterised by its current and voltage. Current can be either direct (DC) or alternating (AC) with a broad variety of voltage (Bonnett, 2005, Eurostat et al., 2009). The main difference between AC and DC systems is the way of transforming and rectifying energy (Bonnett, 2005). While in the case of the DC this takes place at fixed sub stations distributed along the railway lines, in the AC system this occurs in a mobile on board substation (Bonnett, 2005). Most metros, trams and trolleys use the low voltage DC system, while the high voltage AC system is used on the main lines.

Electrification has several advantages compared to diesel traction including: fuel efficiency; reduced environmental pollution; faster acceleration and braking, which is particularly important in the cases of rapid transit; almost unlimited power supply; and lower cost locomotives (Bonnett, 2005, Hoffrichter, 2009). On the other hand, it is characterised by: the necessary high capital cost for its installation; its vulnerability especially in the cases of overhead equipment; and finally its higher maintenance cost.