PATRONAGE on Bangkok's elevated mass transit system, commonly known as the BTS Skytrain, has grown rapidly since it opened in December 1999 so much so that by the end of the 2000s line extensions and signalling upgrades became essential.

With Bangkok Metropolitan Administration (BMA) and Bangkok Mass Transit System (BTS) intent on expanding the system to meet demand as well as increase operational efficiency and redundancy, and optimise maintenance costs, BTS decided to upgrade its signalling system to CBTC and expand its fleet capacity. It subsequently purchased 12 four-car trains from CNR at the end of 2008, which now predominantly serve the Silom Line, while it also extended its existing three-car trains to four-car sets.

The initial expansion of the network encompassed a 2.2km extension of the Silom Line to Wong Wian Yai, which opened in May 2009, and a 5.3km addition to the Sukhumvit Line to Bearing, which opened in 2011. Bombardier secured the signalling upgrade contract in mid 2007, which along with the new trains, was introduced in phases and tied in with the scheduled opening of the line extensions.

The new signalling system consists of Bombardier's CityFlo 450, which utilises computer-based interlockings, radio-based automatic train protection (ATP), a central traffic control system, and a radio transmission and communications network, offering 1min 30sec headways compared with 2 minutes with track circuits.

The new trains were only equipped with the new automatic train control (ATC) system and thus were not compatible with the original signalling system. As a result, installation and commissioning of the new signalling system took place in advance of rolling stock deployment, requiring a seven-phase migration programme:

Phase 1: an interim solution immediately following the Silom Line extension from Saphan Taksin across the Chao Phraya River to Wong Wian Yai. The legacy signalling system controlled the original Silom Line while deployment of the new signalling system took place only on the extension. As a result, some three-car trains were fitted with onboard equipment for both signalling systems.

Phase 2: new ATC trackside equipment and a new central control system was introduced on the entire Silom Line. A few of the new four-car trains progressively entered service on the Silom Line and trains equipped only with new ATC onboard equipment were allowed to operate on the Silom Line in manual ATP mode. No automatic train operation (ATO) operation was available and a fallback to Phase 1 operation remained an option.

Phase 3: more new trains were put into operation on the Silom Line, while a section of the Sukhumvit Line from Chitlom station to Mo Chit was overlaid with new trackside equipment allowing mixed train operation on this section. Trains serving the Sukhumvit Line operated under the original ATC system. The new trains transferred to Silom Line running on this stretch of line operated under the new ATC system in manual ATP mode until transfer to Silom Line where all trains, both the new and original three-car sets, ran on the new signalling system.

Phase 4: ATO operation was introduced on the entire Silom Line.

Phase 5: depot resignalling was conducted with both the original and new signalling systems working in parallel.bangkok-BTS

Phase 6: new trackside equipment was installed on the entire Sukhumvit Line from Mo Chit to Bearing offering full signalling system capability.

Phase 7: progressive decommissioning of the original signalling system, both onboard and trackside equipment.

Inevitably there were significant challenges throughout each of the seven phases, but particularly during the early stages of the project as it took shape.

Work on the initial stages of Phase 1 commenced under the assumption that an interface between the original and the new signalling systems was possible by exchanging the status of the two systems' interlocking. BTS approached the original signalling supplier for a quote to provide this interface. However, one year into the re-signalling project, and following a number of letters and email exchanges, there was no sign that this supplier was willing to offer any assistance.

With time running out to meet the government's scheduled opening of May 2009, the project team and the contractor proposed an interim solution without any interface between the original and new signalling systems. This interim phase would remain in operation until the new signalling system was completed and commissioned on the entire Silom Line.

A study of the station most suitable for the system operation switchover subsequently took place, with the primary focus on achieving the best headway possible. The study also considered layover times to switch from one onboard system to the other, and considered the flexibility of train operation under both normal and degraded operation modes.

Ultimately the study identified Surasak as the ideal location due to reduced layover times at Saphan Taksin, the original line's terminus, and because of the benefits of establishing a definitive "buffer zone" between the old and the new section so that all trains were operational under the new signalling system before they entered the extension. This also permitted early identification of non-communicating trains, which could return westbound with minimal disruption to services while preventing all delays from occurring at a single location.

The buffer zone's signalling system is shown in Figure 1. Balises installed beyond the system operation boundary up to Chong Nonsi enable early identification of non-communicating trains approaching Surasak, while point indicators operate in parallel to the original signals from the fringe of Surasak towards the extension. The new interlocking system provided complete route control from the boundary of Surasak to Saphan Taskin, which were identical to that of the original signalling system.

Bangkok-fig-1Maintaining high levels of safety was inevitably a concern when developing this interim solution, particularly with the need to maintain train operation during the migration period and the relatively short time for implementation. Installation of emergency pushbuttons removed the movement authority and connected both systems in parallel, minimising this risk.

In addition, it is essential to offer continuous radio communication between the train operator and line controller at the control centre. The interface relied on a file transfer from the train management system to the voice radio system using an XML file containing information on active cabin number, vehicle number, train ID, driver number, workstation ID and station ID. The line controller entered the train IDs into the system once a day when the train first entered the system, and again into the legacy system when it crossed into the buffer zone.

As a result, handling train crossings between the two systems required close collaboration between line controllers using the original and new systems. At the control centre both the legacy and new systems control workstations were positioned side-by-side for easy communication between controllers and to facilitate effective train handover. This arrangement also provided straightforward communication between the two line controllers during incident management, which was critical for safe operation in the overlapping control section of track in the event of an emergency if one track was blocked and single-track bi-directional operation was in use. Luckily no such incidents arose during migration.

Following the successful implementation of the interim system, attention shifted to expanding the new signalling system to the entire Silom Line to enable trains to operate in ATP mode. The new system controlled all trackside equipment including points and signals (point indicators) on the Silom Line.

With the Sukhumvit and Silom line interchanging at Siam, the interlocking systems of both the new and original systems required flank protection before a movement authority was possible. As there were no interface connections between the two lines, it was impossible to provide a true interlocking between the two systems. As a result, in order to maintain the fallback option to operate using the legacy system, and with no design information from the original system supplier available, the only option was to explore the possibility of capturing the essential interlocking status from the original system.

The project team and the contractor devised a simple method to achieve this by putting a point detector on the flank points of the other line, all of which were under the control of the original interlocking system with the new interlocking system laid on top but without any control over point operation. Point-loss detection tolerance was set at 1mm, which was smaller than the original system, meaning that any loss in the original system would also result in loss of detection in the new system.

An extension of the new trackside system to Rathchathewi station allowed all trains sent to the Silom Line to operate with the new signalling system. As a result, any non-operative and non-communicating train would continue its service safely on the Sukhumvit Line.

Operation on the Sukhumvit Line remained unchanged apart from locking the points providing flank protection for the Silom Line in a position that does not lead to the other line. The same applied to flank points on the Silom Line but with the original system remaining responsible for point control. As a result, both line controllers must work collaboratively for any train transfer to take place, which involved releasing the point locks and setting cross-line route in the original interlocking system, followed by setting the same route in the new interlocking system.

With only one depot at Mo Chit for both the Sukhumvit and Silom lines, any new Silom Line trains had to transfer from Mo Chit depot to Siam station via the Sukhumvit Line before running onto the Silom Line.

Initially with the introduction of only a few of the new trains to the Silom Line, sets using the Sukhumvit Line in the early morning operated in manual mode, and switched to the new system when they entered the Silom Line. During the off-peak, train stabling took place in sidings, and following the conclusion of the service, the trains returned to the depot in the opposite direction again in restricted manual mode.

However, as more of the new trains entered service, there were insufficient sidings for stabling, resulting in some of the trains returning to the depot. This transfer occurred twice a day - one return trip in the morning peak and again in the evening peak - and the trains had to run safely at speeds on the Sukhumvit Line sufficient to prevent interruption to services. As a result, the northern section of the Sukhumvit Line from Siam to Mo Chit including the turnback tracks used the same approach to cross-line operation. Trackside equipment for the new signalling system was overlaid on this section of track while point detectors were deployed on all points on the Sukhumivt Line's main running line. Apart from route setting sequence, all trains operating in front of the new trains were equipped with the CBTC train position reporting system and were visible so that any new train running on the Sukhumvit Line could operate in ATP mode at line speed.

Following the successful implementation of semi-manual mode, work commenced on the project's fourth phase, which consisted of modifications to the onboard software allowing trains to operate in ATO mode on the entire Silom Line. This was followed by a depot resignalling project from the Mo Chit turnback so that trains using this stretch were compatible with the new and the legacy signalling systems. As a result train movements between Mo Chit and the depot became far more efficient by eliminating the requirement for all new trains to operate in restricted manual mode during transfers.


The penultimate phase consisted of resignalling the entire Sukhumvit Line and providing a connection to the Bearing extension. No specific signalling arrangement was required for this phase of work, as it required just a simple overlay of new trackside equipment on the southern section of the Sukhumvit Line, a connection from the Sukhumvit extension to the Silom Line and northern Sukhumvit Line interlocking as well as the radio network.

With these systems successfully deployed, work could begin on decommissioning the legacy signalling. Careful planning was again required to avoid disruptions to the new signalling system and the teams faced significant challenges, including improving the system's reliability so there were no interruptions to services, and corrections to the temporary works. A software upgrade to the ATP/ATO system, along with a series of high regression and fit-for-operation tests, ensured the system was ready. However, standby engineers were available with a fallback plan if required.

This is one of the few resignalling projects where there were no interfaces between the legacy and new signalling systems during the entire migration. The project shows that it is possible to devise innovative ways to provide the safe movement of trains across system boundaries and on trackside overlaying sections without an interlocking control between systems. This is achievable through barring all conflicting routes to the normal daily operating routes and the appropriate application of the basic signalling principles of route locking in one system and movement authority control in another, while using point detectors to connect the two signalling systems.

This was a difficult project to implement, but its success shows that safe train operation is possible even without the original signalling supplier providing any information to assist with the transition. Achieving this within the timeframe was a team effort and the result of close collaboration among the project team, the contractor and the operator.