INTRO: With Virgin introducing two fleets of tilting trains at 200 km/h from 2002, leading to 225 km/h operation from 2005, Richard Hope reports on the need for systems that will enforce higher speed limits through curves and also prevent tilt where clearances are inadequate
TWO YEARS from now, Virgin expects to introduce its Passenger Upgrade 1 timetable that will see Pendolino Britannico trainsets built by Alstom and Fiat take over West Coast main line inter-city services between London Euston, Birmingham, Manchester and Glasgow (RG 10.98 p707). PUG1 schedules demand 200 km/h operation with tilt. PUG2 sees the line speed between Euston and Crewe raised to 225 km/h from May 2005, giving a further reduction in journey times on all routes.
From May 2001, Virgin’s 34 diesel powered non-tilting Voyager trains built by Bombardier should be in service on several routes, notably between Brighton and Birmingham. They will be closely followed by 44 Super Voyagers (RG 4.00 p222). Together, the two fleets will operate all Virgin’s CrossCountry services as well as London to Holyhead, the only significant non-electrified route in the WCML franchise. Although the Super Voyagers will tilt, none of the fleet is likely to do so before 2002.
One issue yet to be resolved is precisely how Railtrack’s safety standards for tilt will be met. However, two Group Standards that define special train protection measures required to operate with tilt in Britain have now been agreed within the industry. The first, covering differential speed supervision through curves, was issued in December 1999. The second ensures that tilt is locked out when trains pass through tight tunnels and other places where the kinematic loading gauge would be infringed, or use tracks not equipped for tilting.
Within Railtrack’s Safety & Standards Directorate, the Project Manager responsible for identifying hazards and developing Group Standards covering high speed (effectively 225 km/h) and tilt is Ray Metcalfe; he is seconded from consultants Halcrow Rail.
Metcalfe explains that the purpose of Group Standards is to define what functions are required to ensure that the risk posed by tilting trains is no greater than conventional trains.
At the moment, Virgin is the only operator committed to tilt. However, Great North Eastern Railway has more than once proposed 225 km/h tilting trains for the East Coast main line, and is currently bidding for a replacement franchise on this basis. Other tilt operators will doubtless emerge.
Ideally, the system developed to meet these Group Standards on the routes used by Virgin is going to be used elsewhere on Railtrack’s network. Hence the need for a Railtrack Code of Practice to support the Group Standards. To be known as GE/RC8517, the tilt RACoP will incorporate these system specifications as well as other functions such as power supplies for lineside speed signs and arrangements for safe movement of trains when the systems fail.
Group Standard GE/RT8012 sets out the rules for controlling the speed of tilting trains through curves, for which purpose Enhanced Permissible Speeds are established. Up to now, the only visual aid to drivers faced with observing speed restrictions through curves has been lineside signs. Dual speeds for passenger and freight trains are commonplace, and differential limits for different types of passenger train are not unknown.
The principle of relying on the driver suffices in Italy, where speed limits applying to tilting trains are displayed along with the illuminated timetable in the cab and on lineside boards. Where full automatic train protection is available, as in Germany with LZB on lines over 160 km/h, an overlay is usually provided to authorise the higher limit applying to tilt.
When British Rail’s Advanced Passenger Train was developed in the 1970s, C-APT was produced to display enhanced speed limits in the cab using data received from passive balises on the track. Lineside signs continued to show only the limits for non-tilting trains.
Originally, Railtrack proposed that tilt speed supervision, which would slow a train going too fast, should only be required where the speed approaching a curve was dangerously close to the overturning speed in that curve. However, HM Railway Inspectorate’s view was that all enhanced speeds through curves should be supervised because the safety margin against overturning is approximately halved with tilt, typically from 56% of the current limit to around 25%.
So GE/RT8012 requires speed supervision and control at all locations where enhanced limits are introduced for tilt. But unlike C-APT, the higher speed will not be displayed to the driver. This will only occur on routes where there is full automatic train protection with cab signalling.
The reason, says Metcalfe, is that overloading drivers with too much information carries other dangers. Specifically, a driver who had passed a yellow signal, and was suddenly presented with a target speed for an enhanced permissible limite through a curve, might treat it as a movement authority. In other words, he might assume that the red signal around the bend had cleared and he could accelerate to the target speed just displayed.
The Group Standard permits a visual or audible warning to the driver to slow down because the system will cut off power and apply the brakes if he does not. The driver must be told if intervention takes place, and the train data recorder must log the event.
So in the normal way, drivers will only see distinctive lineside signs applying to tilting trains. These must always accompany the limit for non-tilting trains, even if this has not changed, and no more than three speeds in total may be displayed at one location.
An additional complication arises from the fact that Virgin’s Pendolinos are designed to run through curves at 12% cant deficiency whereas the limit for Super Voyagers is 9%. To avoid tilt nausea, the body tilt angle is two-thirds of the cant deficiency, ensuring that passengers sense some lateral force. But it is cant deficiency that determines the safety margin against overturning.
Where Pendolinos and Super Voyagers will share a common route, as between Stafford and Glasgow, lineside signs will display only the higher enhanced speed. A Super Voyager driver will be advised through the Sectional Appendix of the correct speed for his train, but safety is not compromised if he goes through at 12% cant deficiency.
GE/RT8012 requires the speed supervision system to ensure that the train does not exceed the maximum safe speed at the start of the curve. A decision on how this is to be achieved is due shortly.
Where cab signalling and ATP is installed - an explicit requirement over 200 km/h - speed limits displayed to the driver will be those appropriate to the train in question, and the status of its tilt system.
Where there is no ATP, the complications of varying speed limits according to the route set through a turnout are avoided by ensuring that enhanced speeds apply only to one route. Limits applying to any other diverging route are the same as for non-tilting trains. Likewise, there are no enhanced limits where temporary speed limits are in force.
A situation unique to Britain, with its very restrictive loading gauge, is the need to bring coaches to the zero tilt position at locations where the minimum clearance to structures would otherwise be infringed. This includes electrical clearance, including for example an anti-tilt pantograph that has failed in the wrong position.
The close spacing of tracks also means that a failure resulting in coaches being tilted the wrong way could, in a very few specific locations, risk contact with another train.
All such locations - for whatever reason - are designated Tilt Prohibited Sections. There are only around half-a-dozen TPS on the WCML network, so the impact on journey times is minimal.
GE/RT8019 requires tilt operators to provide a Tilt Enable & Supervision System. TESS ’enables’ tilt because it is not considered practical to produce an on-track ’tilt inhibit’ system which is fail-safe. The ’supervision’ element, which is entirely on-train, monitors the active tilt mechanism to ensure that every vehicle returns to zero tilt and stays there.
Tilting trains must operate in non-tilt mode for several reasons, such as the approach to a major station like Birmingham New Street or Euston. They will also be subject to diversions. TESS ensures that the driver cannot activate tilt by mistake in such circumstances, because it will only work when the train has received an ’enable’ authority.
Again, the way in which this is to be achieved is close to agreement. A common system that meets both speed supervision and tilt enable functions is favoured.
The tilt-enable authority expires at the start of the service braking distance needed to comply with the speed limit for non-tilting trains through the TPS. This braking curve is subject to speed supervision. Any speed exceedance that results in intervention, or any detection of failure to achieve and hold zero tilt, results in the train being stopped.
On emerging from a TPS, tilt enable authority will resume. As tilt commands typically migrate down the train from the leading vehicle, the Group Standard only requires that vehicles (not the complete train) ’do not tilt until they have left the TPS.’
Indications to the driver must show whether TESS is operational, failed or isolated, and whether or not tilt is enabled. As with speed supervision, he will be told when the system has intervened to stop the train. TESS functions are comprehensively logged by the train data recorder.
CAPTION: Now taking shape at Alstom’s Washwood Heath plant is the first of 53 Pendolino trainsets for Virgin West Coast services
Photo: Virgin/Milepost 921/2
CAPTION: West Coast’s electric Pendolinos will be allowed to tilt up to 12% cant deficiency, but the CrossCountry diesel Super Voyagers will be limited to 9%
CAPTION: The angled sides of the Voyager DMU cars being built for Virgin CrossCountry by Bombardier in Brugge reflect the tilt capability to be fitted to the Super Voyager derivatives (RG 4.00 p222)
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