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Friday, June 14, 2024

Squeezing more from the tube

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Anyone strap hanging on a crowded Victoria line train could be forgiven for not believing that, a few months ago, its upgrade gave a 21% increase in passenger capacity. The reality is that the various challenging capacity improvement projects being undertaken by London Underground (LU) barely keep pace with London’s insatiable demand for tube travel. This much, and more, was explained by LU’s Malcolm Dobell and Kate Whelan in a recent presentation to the IMechE Railway Division’s Scottish Centre.

To the second operation

Malcolm Dobell, LU’s head of train systems, opened his presentation by outlining the principles of Metro operation. Delivering high capacity needs high acceleration and braking rates (1.3 m/s2 and 1.15m/s2 respectively) and consistency from Automatic Train Operation (ATO) and control of station dwell times. Short headways from lots of signals or moving block signalling is needed as well as lots of wide doors that open and close quickly. The operation needs to be managed to the second, with particular attention to train despatch.

A good example of this ‘to the second’ operation is the need to turn around trains in 218 seconds at Brixton on the Victoria Line. In this time a driver shuts down his cab, a different driver opens up the cab at the other end, passengers have to leave and board the train, a route is set over the crossovers and the train driven clear of the platform. It has been assessed that this requires at least 203 seconds.

Upgrading an automated tube

This 218 second turn around is needed for the new 33 trains per hour (tph) timetable introduced in February with the completion of the Victoria line upgrade. The increase from the previous 27 trains per hour comes from the provision of new signalling and rolling stock to give a 21% capacity increase. However, in the time taken to upgrade the line, ridership has increased by 25%, from 160,000 to 200,000 passengers per day.

When it opened in 1968, the Victoria line was the world’s first full-scale Automatic Train Operation (ATO) railway. The recent ATO-on-ATO upgrade replaces the original fixed block system with Westinghouse’s Distance to Go signalling as explained in The Rail Engineer issue 98 (December 2012). As this new system overlaid the old signalling, it was installed largely without disruption.

At 133.3 metres, the 2009 Victoria line stock forms LU’s longest deep- tube trains – three metres longer than the 1967 stock it replaced. This uses the extra platform length that had been provided due to initial uncertainty about the original ATO stopping accuracy. It is also 40mm wider than normal tube stock, to take advantage on the line’s slightly wider tunnels. The 47 new eight-car trains were built by Bombardier, 39 are required to provide the service.Brixton turnaround [online]

Extra train capacity serves no purpose if passengers can’t get to them, so an essential part of LU’s capacity improvement programme is made up of station enhancements. The recent addition of the northern ticket hall at Kings Cross tube station is one example, as are various ongoing enhancements including those at Victoria underground station. Here, in 2018, a new booking hall under Bressenden Place and nine new escalators are to be provided.

The energy dissipated by extra trains adds to the problem of cooling the tube. LU’s approach to this issue is described in issue 98 of The Rail Engineer (December 2012) which also provides more information about Kings Cross tube station.

Modelling the future

Kate Whelan, LU’s lead systems performance engineer, has a love of modelling. This much is clear from her enthusiastic description of LU’s railway engineering simulator (RES), a bespoke application developed by LU which models system operation (including rolling stock, signalling, track and power) to a high level of detail. For her, the RES is an essential tool to ensure optimum system performance.

In her presentation, Kate described how the RES translates business and operational requirements into engineering projects and is then used to verify and validate project design proposals in order to deliver the required timetable. As an example, RES has been used to specify the remodelling of Baker Street as part of LU’s Sub-surface Lines (SSL) upgrade (The Rail Engineer issue 85 – November 2011). Currently, 14 trains per hour (tph) run from the Metropolitan line platforms over the flat Baker Street junction, meshing with 12 tph from the Circle Line

to give 26 tph through the junction. In addition 8 tph from Harrow terminate at Baker Street. After the upgrade, the train service will be respectively 16 and 16 tph (32 tph through the junction) with 12 tph terminating at Baker Street.

An extra 65%

Transport for London’s leaflet ‘Our Upgrade Plan – Improving London’s Underground’ gives an overview of all LU’s upgrades. It shows that by far the greatest peak capacity improvement is an impressive 65% by 2018 for both the Circle and Hammersmith & City lines. These form part of the SSL network, the other lines being the Metropolitan and District for which capacity improvements of 27% and 24% respectively are planned.

The SSL network has 310 track kilometres, of which 85% is actually on the surface, with multiple flat junctions requiring inter-meshing of the different lines service timetables. It carries 1.3 million people a day, 25% of LU’s ridership. Part of its network is also shared with the Jubilee and Piccadilly lines as well as ‘heavy rail’ (Chiltern and South West Trains). So whilst its current operations are challenging, a 65% increase in capacity requires a quantum change. This requires a system performance engineering approach using RES to model all aspects of the system including new trains, new signalling and train regulation systems, control centres, track upgrades, changed layouts and power upgrades.

Enter the S stock

For passengers, the first obvious improvement is the new S stock with its air conditioning, a first for LU. The £1.5 billion order for Bombardier to supply 191 new S stock trains between 2010 and 2016 is said to be the largest in UK railway history. It comprises of 58 eight-car S8 units for the Metropolitan Line and 133 seven-car S7 units for the other sub-surface lines. The S7 and S8 stock trains are respectively 24 and 2 metres longer than their predecessors and have through gangways and revised seating. Their introduction gives an immediate capacity improvement (25% for the Circle Line).

Prior to the introduction of this stock, platform extensions were required, although at stations where this was prohibitively expensive, selective door operation is used. To cope with the additional air conditioning load and the eventual performance upgrade, power supplies were improved with high-conductivity stainless steel/aluminium conductor rails. Further supply upgrades will be required for the increase in train frequency prior to commissioning the new signalling system in 2018.

CITYFLO – a quantum leap

RES Baker St [online]

RES modelling trains through planned new layout at Baker Street.

As has been seen, the system wide approach to the SSL upgrade involves many projects, for example the renewal of 36 junctions between 2015 and 2018. However it is the new train control system that offers the greatest capacity improvement. This is Bombardier’s CITYFLO 650 system for which a £354 million contract was awarded in June 2011. CITYFLO 650 is a variable moving block Communications Based Train Control (CBTC).

The system has four elements: Automatic Train Protection (ATP), Automatic Train Operation (ATO), Train to Wayside Communications (TWC) and Automatic Train Supervision (ATS). Train location is determined from a combination of tagged base stations and axle-driven tachometers. The system has no external signals, track-circuits or trip cocks. Therefore, unlike most re-signalling schemes, it can be installed with minimal impact to the existing signalling system.

ATP has train and trackside components to perform safety critical tasks such as train detection, safe train separation and junction control. ATO ensures the train is operated to a defined speed profile and stops accurately. The TWC provides secure radio communications between all elements of the system via leaky coaxial cables or line of sight antennas. A significant level of redundancy is built into all these systems.

With its ATS functionality, CITYFLO 650 is far more than just a signalling system. ATS actively manages the train service in accordance with the operator’s requirements. This includes start-up, changes to service frequency and the management of headways. Crucially it controls train movements to ensure there is no conflict as trains arrive at the junctions which currently constrain the capacity of the SSL network. The Rail Engineer issue 85 (November 2011) has more information about CITYFLO 650 and the SSL upgrade.

It all sounds quite wonderful, but then so did a similar signalling scheme proposed as a solution for the problems of the West Coast main line 20 years ago. In this case, however, there is a proven system which is already in successful operation on 14 networks, including Madrid’s lines 1 and 6 and Shenzen’s L3 metro. Another factor is that LU has procured CITYFLO ‘as is’ and, where necessary, is modifying its processes to accommodate it. Other risk mitigation is extensive test track testing and RES simulation.

Lessons for main lines

In his presentation, Malcolm considered whether main line railways could benefit from applying the principles of metro operation. For metros, train separation is typically less than 300 metres at 50 km/h compared with over 2000 metres at 160 km/h. Main line has mixed traffic with varying traction and rolling stock (T&RS) performance and stopping patterns all of which reduce capacity. As a result, main lines do well to achieve 12 tph compared with a metro’s typical 30 tph.

Despite these differences, he felt there were transferable lessons. Metros can only achieve their train service by detailed attention to every operational aspect, requiring a production line approach. Metros do not have the problem of mixed traffic which can be alleviated by attention to dwell time and consistent T&RS and stopping patterns. Whilst it is evident that infrastructure improvements such as passing places and flyovers will also increase capacity, these needed to be modelled at an engineering level to optimise design as LU do with their RES software.

150 years and going strong

This year LU is 150 years old. From carrying 60,000 passengers in 1863, there are now over a billion journeys a year. Over this time, ‘The Underground’ has expanded and adapted itself to take this extra traffic which will continue to increase with 1.3 billion journeys forecast for 2020. Accommodating this extra traffic through the small tunnels and over complex flat junctions that are part of LU’s heritage is a significant challenge for which it has solutions.

With today’s safety regime, running a steam train that ran over the original Metropolitan lines to celebrate its 150th birthday was a significant achievement. This shows the same can-do philosophy and attention to detail that LU applies to both its day-to-day operations and upgrade projects. So it is good to see that one of the details of the SSL upgrade is the requirement to fit CITYFLO equipment to LU’s heritage trains.

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