Article
citation information:
Irlik, M. Train headway optimisation using virtual blocks. Scientific Journal of Silesian University of Technology. Series
Transport. 2021, 112, 75-84. ISSN:
0209-3324. DOI: https://doi.org/10.20858/sjsutst.2021.112.6.
Maciej IRLIK[1]
TRAIN
HEADWAY OPTIMISATION USING VIRTUAL BLOCKS
Summary. Although the
implementation of a new Automatic Train Protection (ATP) system increases the
safety level, it also exerts some impact on the blocking time in block
sections, and consequently, on the headway between trains. At the same time,
ATP systems introduce a train positioning system based on odometry
calculation and reporting back to the trackside system. This paper describes
the concept of using virtual blocks based on train position reporting in the
ATP system for purposes of non-occupancy determination. Virtual blocks can be
used to reduce headways on railway lines without increasing the number of
trackside signalling devices. Preliminary capacity assessment was performed to
calculate the average headway depending on the signalling system with reference
to a case study.
Keywords: railway, signalling, ETCS,
ATP, capacity
1. INTRODUCTION
Automatic
Train Protection (ATP) systems based on track-to-train radio communication use
continuous transmission in the train control process for sending movement authorisations
to trains. In this process, trains provide the
trackside equipment with reports on their position relative to fixed markers (balises) distributed along the track and reflected by the
geographic arrangement of the trackside railway traffic control devices. An
example of an ATP system featuring track-to-train radio communications is the
European Train Control System (ETCS), implemented in
Poland with an intent to replace the legacy system - Automatic Train Braking
System (Samoczynne Hamowanie
Pociągu, SHP).
ETCS provides a harmonised description of the data exchanged between the
trackside and on-board devices, and of the behaviour of the on-board equipment
in terms of safe control of the train movement. For
the sake of train movement authorisation, the system retrieves route related
data from railway traffic control devices and allocates them to a specific
train. In Poland, minimum headways are harmonised and depend on the adopted
train braking distances from a specific running speed to a stop [1]. This approach results from the
implementation of ETCS over the existing
infrastructure as well as the need to manage mixed traffic operations, that is,
operations of trains equipped and not equipped with the ETCS
devices.
Track-to-train radio communication makes it possible to control train
traffic based on moving block sections, where train detection relies only on
the reported train position, and where granting movement authorisation is
relative to the end of the preceding train.
Consequently, the authorisation end may be located at any point of the
infrastructure [1]. The reason this solution additionally comprises conventional
detection systems is that it enables the detection of unauthorised movements
and makes it easier for a system to return to normal operation after being
temporarily unavailable.
The solution described in this paper involves an
additional division of traditional block sections into virtual blocks whose
state of non-occupancy would result from a logical state depending, among other
aspects, on the position reported by a train. The N number of such virtual blocks may form a real block section
featuring – at its beginning and end – conventional occupancy
detection systems. According to this
solution, the end of the movement authorisation can only be set at certain
fixed locations.
The purpose of this paper is to present the train traffic control
process based on virtual blocks and to conduct a preliminary assessment of the
effect of this solution on the railway line capacity using a chosen railway
station as an example.
2. STATE OF THE PROBLEM AND
LITERATURE REVIEW
Insufficient railway line capacity translates into the necessity of
either limiting the rail transport offering or re-routing trains on alternative
lines, which may cause the rail transport to cease being competitive compared
to road transport. An aspect, which is considered very significant for the
development of rail transport, is its low-carbon nature compared to road
transport, as well as its lower overall negative environmental impact [2-4]. The traffic operated in the Polish railway network, where
the ETCS Level 2 is currently in operation, is of a
mixed character, which applies to trains both equipped and non-equipped with
on-board ETCS devices. Such a traffic structure may
increase its heterogeneity due to the difference between the train speed
supervision based on the braking characteristic calculated by the ETCS (commonly referred to as a braking curve) and the
driver’s driving style and behaviour vis-à-vis the trackside
signalling equipment. This, in turn, can lead to an increase in the headway
time and distance between two consecutive trains. As evidenced in paper [8],
heterogeneity is determined by the index of homogeneity of blocking time, the
homogeneity in terms of time buffers between consecutive trains, and the
homogeneity in terms of the direction of movement of individual trains.
Fig. 1 illustrates sample braking characteristics according to the model
applied under ETCS, established using the ETCS braking curve model, as described in specification
[7]. The model train used for the simulation is characterised by the share
of real braking mass used for emergency braking of λ0
equalling 140 [%], which corresponds to the minimum value required for a
braking distance of 1,300 [m] (assuming the ruling gradient of 0 [‰])
according to the traditional approach, and as per Appendix 1 to the Ir-1 manual [5, 6]. The simulation result thus
obtained implies that ETCS will instruct the driver
in terms of the need to apply braking at a distance of 2,617.75 [m] from the
authorisation endpoint, while if there is no response, the system will respond
by initiating full service braking at 2,035.22 [m] before the place where the
authorisation ends.
Fig. 1. Braking curves calculated by Braking
curves simulation tool v 4.2 for the model train [author’s study]
The literature extensively describes the models applied to assess the
capacity of railway lines, consequently used in both simulation and analytical
methods [9-14]. An assessment comparing simulated and real routes of a train
equipped with the ATP system was discussed in the work [15]. An example of the
impact exerted by an extended braking distance (Fig. 1) on the characteristics calculated by ETCS
relative to the traditional minimum block section lengths are presented in Fig. 2 using the model described in paper [9] for blocking of individual block
sections. The longer braking distance calculated by
the ETCS system based on the assumed division into
block sections may cause the given block to be occupied for a longer time or
make it necessary to extend the headway. Where an opposite situation is the
case, and the block section is longer than the braking distance calculated by
the ETCS system, the headway may be shorter than that
which applies to operations under a traditional signalling system.
The literature refers to various ways to increase railway capacity using
various technical or optimisation solutions. The research [16] discusses how
the length of block sections can be chosen automatically for purposes of the ETCS. Using an algorithm for the
dynamic optimisation of train headway time was described in the study [17].
An option of additionally increasing the railway line capacity if moving block
sections are used was discussed in paper [1818], arguing that a relative braking distance, as it is commonly referred
to, should be considered in the process of separating consecutive trains in
movement. Where this is the case, it is assumed that
the preceding train, even if it applies emergency braking, will continue to
move because of its inertia. According to this approach, the authorisation
granted to the next train can account for the foregoing, making it possible to
reduce the headway distance and time. Such a solution requires an additional
infallible channel of communication between trains. The research [19]
speaks of reducing the headway between trains even more, suggesting that one
should apply a virtual coupling, that is, coupling two consecutive trains
virtually for their control process. In solutions
such as those proposed, using even more reliable communication technologies and
developing methods for even more accurate train positioning seems to be the key
problems.
Fig. 2. Effect of a more restrictive braking curve in the ETCS
against traditional headways, where a corresponds
to train deceleration and b to train
acceleration
[author’s study based on [9, 10]]
3. CONCEPT OF VIRTUAL BLOCKS
The concept of virtual blocks (VB) implies additional division of fixed
block sections for ETCS trains. In order not to increase the quantity of the trackside
equipment, it has been proposed that this division should be based on VB, which
do not require traditional non-occupancy detection systems.
The set of n VB would comprise
a fixed block section as per the traditional approach, that
is, terminated by physical non-occupancy confirmation devices. The logical state of such a virtual block could be defined
as a logical sum of:
• the physical state of non-occupancy of a fixed block that is
part of the locked route,
• the train location as derived from its reported position.
Such a solution could make it possible to decrease the headway
separating two consecutive ETCS trains in movement
and give the possibility to operate non-equipped with the ETCS
devices in trains. The concept of VB is illustrated using the example shown in Fig. 3, where an additional division of a three-aspect block signalling system
featuring light signals has been proposed.
Fig. 33. Signalling system layout with 3-aspect signalling (line block) and VB
[author’s study]
Besides developing the block division on the route, another viable
alternative is to apply this solution within station areas so as not to create
bottlenecks, as they are commonly referred to. For example, it is possible to
break down an outbound route from the station, since its length is
traditionally much longer than the braking distance.
4. PRELIMINARY CAPACITY ANALYSIS (CASE STUDY)
The model setup used for purposes of
capacity assessment with the application of the virtual block solution is the
outbound route from the Bolesławiec (Bc) station towards the Bolesławiec-Zebrzydowa (Zb)
route (Fig. 4). In this case, the lengths of the block sections (including the
outbound route) are significantly longer than the typical braking distance
assumed for the speed of 160 [km·h-1]
equalling 1,300 [m].
Fig. 4. Example of the signalling system layout at the Bolesławiec
(Bc) station with the proposed
additional VB on the outbound route towards the Zebrzydowa
(Zb) station
[author’s study]
Table 1 and
Fig. 5 show the minimum headway times established for the train analysed in
the case study for the following conditions:
·
the
outbound route is split in real life into fixed block sections where the train
operations are managed by three-aspect trackside signalling equipment;
·
ETCS Level 2 implemented at the existing block sections;
·
VB in use.
The data analysed in this case study
concern movement without stopping. The proposed division into VB covers the
outbound route from the P/Q exit signals as long as to the new movement
authorisation end markers installed near the U/W entry signals (Figure 5). The minimum headway times, given by relationships
(1–3), have been determined as follows:
(1)
(2)
(3)
where: tH1
is the minimum headway time for two consecutive trains running in traditional
block sections, tH2 is the minimum
headway time for two consecutive trains in the case where the ETCS determines the beginning of braking until the
authorisation end, tH3 is the
minimum headway time where VB are used, SD is the signal sighting
distance on a given maximum line speed, LBlock
is the length of m-1 blocks between
trains (m denoting the number of
aspects of the signalling system), doverlap
is the overlap length, LTRAIN_2 is
the length of the preceding train, tIL
is the time of actuation of the traffic control equipment, v is the
running speed of train “2”, and BC
is the length of the braking distance to the target according to the Permitted speed braking curve for the
given train “1”.
Tab. 1
Computational
headways for a train running on an outbound route from the Bc station [author’s study]
|
Speed v [km·h-1] |
Sighting distance SD [m] |
Braking curve BC [m] (ATP) |
tH1 for 3-aspect signalling [s] |
Theoretical tph for 3-aspect signalling [train·h-1] |
tH2 for fix blocks with ETCS
[s] |
Theoretical tph for fix blocks with [train·h-1] |
tH3 for VB with ETCS
[s] |
Theoretical tph for VB with ETCS
[train·h-1] |
|
40 |
400 |
420 |
430.05 |
8 |
207.00 |
17 |
84.30 |
42 |
|
80 |
400 |
788 |
216.53 |
16 |
122.31 |
29 |
63.96 |
56 |
|
100 |
400 |
831 |
173.82 |
20 |
100.30 |
35 |
54.82 |
65 |
|
120 |
400 |
1,106 |
145.35 |
24 |
92.58 |
38 |
55.68 |
64 |
|
140 |
467 |
1,422 |
126.73 |
28 |
88.12 |
40 |
58.21 |
61 |
|
160 |
533 |
1,850 |
112.76 |
31 |
87.30 |
41 |
62.63 |
57 |
The value of time tIL was calculated for a time starting
from the physical block section release after the passage of train
“2” until the moment of enforced locking of a new route for train
“1” and setting the signal at clear for this train. tIL_ETCS is the time of actuation of the
railway traffic control equipment, considering not only the time of releasing
and enforced locking of a new route, but also the time of a new train
authorisation being generated and read by the on-board equipment along with
updating the supervised new end of movement authorisation for train
“2”, dVB is the virtual
block length, and TCYCLOC
corresponds to the cyclic nature of the train position reporting.
Fig. 5 Results of calculated min. headway times for
the signalling concepts subject to analysis [author’s study]
Based on the results of the minimum
headway time calculation, the author also determined the theoretical number of
trains that could be sent out per hour using a specific solution, assuming
different maximum line speeds (Figure 6).
Having analysed the results thus
obtained, it should be noted that in the case of the additional division of the
outbound route from the station and the first block section, it is possible to
increase the theoretical number of trains equipped with the relevant on-board
equipment per hour from 25 [trains per hour] (at 40 km/h of running speed) to
16 [trains per hour] (at 160 km/h). Moreover, where
this is the case, it is easy to recognise the benefit of using ETCS, even based on the traditional and fixed division of
the outbound route and the first block section, which results from the length
of the main set route from exit signals P/Q to the first blocking signals 388N/387, which is longer than the braking curve calculated
by the ETCS.
5. APPLICABILITY DISCUSSION
Increase of the line capacity with the traffic control using virtual
blocks requires that most of the trains will be equipment with additional
device for train integrity monitoring and the entered to the system train
length is confidence and validated. The purpose of this is to provide to
trackside part of the system the confident location of the rear of the train
and that the train is complete. Management of this device as well as putting
correct data related to the train length require high safety culture in train operator company nevertheless should not base only on human
factor. Especially for non-multiple-unit trains, the train length already
entered by driver, should be validated e.g. by additional plausibility check
compared to amount of carriages or by axle counters to confirm the train length
in places where the trains make start of mission.
Fig. 6. Calculated number of trains per hour depending on speed for the case
study analysed [author’s study]
6. CONCLUSIONS AND FUTURE WORK
This paper discusses
the process of train traffic control based on VB and addresses a preliminary
assessment of the effect of this solution on the railway line capacity.
Under the preliminary assessment of
the effect of the solution with virtual blocks on the railway line capacity
parameters, the theoretical minimum headway times and the theoretical number of
trains per hour were calculated for the chosen station’s outbound route. The calculation results thus obtained confirmed the
capacity increase achieved using virtual blocks compared to the application of ETCS with the traditional block section division and
against train operations using the trackside signalling equipment (in this
case, the assessment was performed only for running speeds up to 160 km/h).
While maintaining the
existing block section division based on conventional non-occupancy detection
methods, the presented solution makes it possible to perform mixed traffic
operations, that is, to operate trains both equipped and non-equipped with ETCS.
The author implies
that further research on the proposed solution should comprise establishing a
simulation environment where results can be obtained for different train types
and for a railway line section covering at least a station and a route. Such a simulation should consider the train movement
dynamics because of the variable speed profile and train stopping either at a
station for maintenance purposes or at an on-route stop.
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Received 05.03.2021; accepted in revised form 03.06.2021
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