Light Rail for better public transport
Metrolink Introduction > Signalling: updated 8 December 2011
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Light Rail Transit Association
Light Rail for better public transport |
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Metrolink Introduction > Signalling: updated 8 December 2011
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Metrolink’s Altrincham and Bury lines have a railway like signalling system while the city centre street running section has line of sight running.
A new Tram Management System (TMS) is required to increase network capacity for the new extensions also to provide both passenger and operational benefits. This TMS is based on line of sight operation, however, it is different from the present line of sight in the city centre.
To switch the system switch over all at once would require Metrolink to be shut down for a considerable period to implement the changes and also to train all the drivers, control room staff and maintainers.
Transition from the existing signalling is being staged. This requires integration with both existing fixed block 2 aspect signalling and the original city centre line of sight system. A custom designed arrangement is required at each interface between old and new systems. Technical difficulties with this work have slowed the TMS introduction, delaying new line openings. Eventually the whole network will be converted to TMS.
There are two distinct types of signal for Metrolink. On the former rail lines and similarly segregated sections two aspect red and green signals and track circuit block working is used. For the street running sections, where vehicles are driven on sight, a white five light signal is used.
This is used to send information from the vehicle to both Metrolink Control and the street running signal system. The VRS equipment generates a modulated carrier signal which is transmitted by a pair of coils, mounted one after the other under the unmotored centre bogie. The VRS signal is picked up by cable loop, buried in the road surface or mounted on the sleepers of segregated track.
At the start of each journey the driver enters a route code into the VRS control panel. This sets up the description of the tram on the diagram at Metrolink control; it also, if required, calls for the points to change on street running sections. The “ready to start” request is also sent by the VRS.
Former heavy rail sections were totally resignalled. The Altrincham line had four aspect colour light signals and track circuit block (TCB) working. The Bury line had both colour light signals with TCB and semaphore signals with absolute block working.
Metrolink has TCB working with a two aspect (red/green) signals, mostly sited at station platform ends. Additional signal sections are provided where traffic levels will justify, such as between Cornbrook and G–Mex. Repeating yellow/green signals are only provided where there is a restricted view of stop signals. Signals and points are interlocked on segregated sections, similar to railway operation.
The braking distance for light rail vehicles is so short that there is no need for distant signals in the main line railway sense. Train stops are provided at stop signals. If a vehicle passes a stop signal at red, even under instruction from control, the train stop causes an emergency brake application using the electromagnetic track brakes. Even at very slow speed this is so sudden a deceleration that drivers announce the event warning passengers to brace themselves.
Metrolink controls all the segregated section signalling except that from just south of Timperley to Altrincham inclusive. Here trams run alongside main line trains and over two level crossings under Network Rail control.
On street running sections tram signals are integrated with the road traffic signals. To differentiate between tram and general traffic signals a new form of signal head was devised. This consists of a cluster of white lights, which are lit according to the indication required.
Five lamps are lit for each aspect, any two failing will not obscure the display’s meaning. A horizontal line indicates “stop”, a vertical line “proceed”, a diagonal line “proceed to left or right” and a “+” sign cluster “stop if it is safe to do so”.
Trams are driven on sight. The proceed indications permit a driver to pass if the road is clear of obstruction, they do not indicate it is clear as green signals do on the segregated sections.
As the tram passes over detection loops buried in the road, the VRS requests a tram “proceed” indication from the road traffic signal Urban Traffic Control System.
When ready to depart from a stop and cross a road the “ready to start” request is also sent by the VRS.
Facing points at junctions are called to change position, if required, by a signal from the VRS. The control circuits only allow point movement when the tram driver can see them move, they also prevent movement when a tram is moving over the points.
Street running signals are not interlocked with facing points at junctions. A separate seven light fibre optic indicator is used to show that the points are set and locked. The indication is a diagonal line of white lights indicating left or right route. If the points are not detected as set in the direction called then the point indicator shows a horizontal white line for stop.
Facing junctions on the system are a choice of one route from two. Both point indicator and tram traffic signal show a matching diagonal indication.
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This page was written by Tony Williams, Manchester Area Officer, Light Rail Transit Association. Contact manwebm@lrta.org if you have any comments, ideas or suggestions about these pages.