1
Chapter 1
Introduction to
Communications-Based
Train Control
Li Zhu, F. Richard Yu, and Fei Wang
1.1 Introduction
Rapid population explosion has resulted in a series of problems, such as trac jam,
environment pollution, and energy crisis. Recently, there has been a strong desire
around the world to improve the rail transit speed and capacity in order to relieve
the pressures from already-busy roads to address the need for fast, punctual, and
environmentally friendly mass transit systems.
e train signaling systems need to evolve and adapt to safely meet this increase
in demand and trac capacity [1]. e main objective of communications-based
train control (CBTC) signaling system is to increase the capacity by safely reducing
Contents
1.1 Introduction .................................................................................................1
1.2 Evolution of Train Signaling/Train Control Systems ....................................2
1.3 Main Features and Architecture of CBTC Systems ......................................5
1.4 Challenges of CBTC Systems .......................................................................7
1.5 Projects of CBTC Systems ............................................................................8
1.6 Conclusion .................................................................................................13
References ...........................................................................................................13
2 ◾ Advances in Communications-Based Train Control Systems
the time interval (headway) between trains traveling along the line. Specically,
CBTC makes use of the communications between the railway track equipment
and the train for train control and trac management. Because the exact position
of a train is known more accurately than with the traditional signaling system, the
railway trac can be managed more eciently and safely.
As dened in the IEEE 1474 standard [2], a CBTC system is a “continuous,
automatic train control system utilizing high-resolution train location determina-
tion, independent of track circuits; continuous, high-capacity, bidirectional train-
to-wayside data communications; and trainborne and wayside processors capable
of implementing automatic train protection (ATP) functions, as well as optional
automatic train operation (ATO) and automatic train supervision (ATS) functions.”
In this chapter, we rst present the background and evolution of train signaling/
train control systems. en, we introduce CBTC systems, followed by the main
CBTC projects around the world.
1.2 Evolution of Train Signaling/Train Control Systems
e main objective of a train signaling/train control system is to prevent collisions
when trains travel on the railway track. erefore, a common ingredient of various
types of train signaling systems is as follows: the locations of the trains must be
known by the system at some level of granularity.
e rst generation of train control architecture includes track circuits for train
detection, wayside signals to provide movement authority indications to train oper-
ators, and trip stops to enforce a train stop [1]. Figure1.1 illustrates this architec-
ture. In Figure1.1, if track circuit TC5 is occupied (shunted by a train), the signal
at the entrance to TC5 displays a red aspect. If block TC3 is unoccupied and TC5
is occupied, the entrance signal to TC3 displays a yellow aspect. If both TC1 and
TC3 are unoccupied, the entrance signal to TC1 displays a green aspect. ese
signals are separated by the train’s safe braking distance (SBD), which is calculated
and set at a sucient length for a train to stop safely from the maximum operating
speed specied for the track section. We can see that, in this system, a green aspect
means that two blocks (or at least twice SBD) are clear ahead of the signal; a yellow
aspect means that one block (at least SBD) is clear ahead of the signal; and a red
aspect means that the block ahead has a train occupying the track circuit.
TC1TC3 TC5
Figure1.1 Train signaling system using wayside signals.
Introduction to Communications-Based Train Control ◾ 3
is simple train signaling system is similar to road trac light signaling systems.
Due to its simplicity, this train control philosophy has served the industry well and
continues in service operation at many major train transit systems around the world.
Nevertheless, with this train signaling architecture, the wayside logic does not know
the exact location of the train in the track circuit. Instead, it only knows that the
train is located somewhere in the block. Because the blocks cannot move, this kind
of systems is known as xed block systems. e main drawback of this system is that
the achievable train throughput and operational exibility are limited by the xed-
block, track circuit conguration and associated wayside signal aspects.
e next evolution in train signaling was also track circuit-based with the way-
side signals replaced by in-cab signals, providing continuous ATP through the use of
speed codes transmitted from the wayside through the running rails to the train. Such
coded track circuits were developed by signaling suppliers in the United States around
the middle of the last century. Although they were not immediately applied to transit
railways, they ultimately made a signicant contribution to the next- generation train
control systems. With this train control architecture, a portion of the train control
logic and equipment is transferred to the train, with equipment capable of detecting
and reacting to speed codes, and displaying movement authority information (per-
mitted speed and signal aspects) to the train operator. is generation of train control
technology permits automatic driving modes, but train throughput and operational
exibility are still limited by the track circuit layout and the number of available
speed codes. is generation of signaling technology entered service in the latter half
of the twentieth century, including the Washington (WMATA), Atlanta (MARTA),
and San Francisco (BART) systems in the United States, the London Underground’s
Victoria Line, and the initial rail lines in Hong Kong and Singapore. Many rail transit
agencies also adopted this technology in order to transit ATO with continuous ATP,
such as London Underground’s Central Line resignaling.
e next signicant evolution in the train signaling architecture continued the
trend to provide more precise control of train movements by increasing the amount
of data transmitted to the train such that the train could now be controlled and
supervised to follow a specic speed–distance prole, rather than simply respond-
ing to a limited number of individual speed codes. is generation of train control
technology, also referred to as “distance-to-go” technology, can support automatic
driving modes and improve train throughput. Under this train control architec-
ture, the limits of a train’s movement authority are still determined by track circuit
occupancies, as illustrated in Figure1.2.
e wayside processor in Figure1.2, knowing the location of all trains via track
circuits, can generate coded messages to each track circuit. is information con-
tains the permitted line speed, the target speed for the train, and the distance-to-go
to the target speed. Using this information, the train’s onboard equipment calculates
the speed–distance prole for the train to follow. In addition, a track map database
with grade, curvature, station location, and civil speed limit information is stored
within each train’s cab signaling equipment. e train knows which track circuit it
4 ◾ Advances in Communications-Based Train Control Systems
is in via a unique ID of the cab signaling information. e cab signaling equipment
then uses the track map database to calculate the accurate speed–distance prole.
e next generation of the train control architecture is generally referred to
as CBTC. e goal of a CBTC system is the same as the traditional systems,
forexample, safe train separation; however, it also has the challenge of minimizing
the amount of wayside and trackside equipment. is means the elimination of
traditional train detection devices, that is, track circuits. Similar to the previous
generations of train control technologies, CBTC supports automatic driving modes
and controls/supervises train movements in accordance with a dened speed–
distance prole. For CBTC systems, however, movement authority limits are no
longer constrained by physical track circuit boundaries but are established through
train position reports that can provide for “virtual block” or “moving block” con-
trol philosophies, as illustrated in Figure1.3.
Wayside processor
Track circuits detect presence
or absence of trains within
the fixed track circuit blocks
Movement authority
Movement authority
ATP profile
Civil speed
Location
Figure1.2 Prole-based train control system.
Wayside processor
Track circuits may be
retained for broken rail and
secondary train detection
Movement authority
Movement authority
ATP profile
Civil speed
Location
Some wayside signals ma
y
be retained for the
protection of unequipped
/
failed trains
Movement authority
Location
Figure1.3 CBTC system.
Introduction to Communications-Based Train Control ◾ 5
In CBTC systems, a major portion of the train control logic is now located
within the train-borne CBTC equipment, and a geographically continuous train-
to-wayside and wayside-to-train data communications network permits the trans-
fer of signicantly more control and status information than is possible with the
earlier generation train control systems. As such, CBTC systems oer the greatest
operational exibility and can support the maximum train throughput, constrained
only by the performance of the rolling stock and the limitations of the physical track
alignment. In particular, the high level of control provided by CBTC systems makes
this the technology of choice for driverless/unattended train operations (UTOs).
1.3 Main Features and Architecture of CBTC Systems
With the exact location information of a train in CBTC systems, the following
train can follow up the rear of the train with a moving block system. Specically,
the train location and its braking curve are continuously calculated by the trains,
and then communicated to the wayside equipment. en, the wayside equipment
is able to establish protected areas, each one called limit of movement authority
(LMA). In addition, CBTC systems use closed-loop control between the train and
the ground control center to improve the reliability of train control. Consequently,
this results in a reduced headway between consecutive trains and an increased
transport capacity. Moreover, using digital radio transmissions, CBTC can achieve
two-way large-capacity communications between the train and the wayside, which
can reduce unnecessary train acceleration and deceleration braking, improve pas-
senger comfort, and enable signicant energy savings.
CBTC systems allow dierent levels of automation or Grades of Automation
(GoA), as dened and classied in IEC 62290-1 [3]. e grades of automation avail-
able range from a manual protected operation, GoA 1 (usually applied as a fallback
operation mode), to the fully automated operation, GoA 4 (UTO). Intermediate
operation modes comprise semiautomated GoA 2 (semiautomated operation [STO]
mode) or driverless GoA 3 (driverless train operation [DTO]). e latter operates
without a driver in the cabin but requires an attendant to face degraded modes of
operation as well as guides the passengers in the case of emergencies. Please note
that, although CBTC systems are considered as a basic technology for “driverless”
or “automated trains,” it is not a synonym for them.
e typical architecture of a modern CBTC system consists of the following
main components, as shown in Figure1.4.
1. Wayside equipment. It includes the interlocking and the subsystems control-
ling every zone in the line or network (typically containing the wayside ATP
and ATO functionalities). e control of the system is performed from a com-
mand ATS, though local control subsystems may also be included. Depending
on the suppliers, the architecture may be centralized or distributed.
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