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January 26th 2010

Madrid's underground railway, like the world's other major underground systems, has grown steadily over time. When it first opened in 1919, it had eight small stations and 3.5km of track running in narrow tunnels. Enlargement and extension soon followed and, by the outbreak of the civil war, there were three main lines and a branch. These served a useful purpose as air raid shelters during periods of hostility.

With the war over, a fourth line was added in 1944. Work continued throughout the sixties and seventies, although the eighties saw a break in construction. Nevertheless, ten lines were in operation by 1989 and further development was continuing. A major expansion over the last ten years has seen Line 8 run out to the airport and over 40km of tunnels and 28 new stations opened in 2003 as part of the MetroSur project.

Today the network is run by Metro de Madrid. It has 12 lines and a branch covering a distance of 284km with 241 stations. This makes it the third longest metro in Europe although it serves a city of only six million people. Unsurprisingly, such an extensive system attracts heavy usage - 76% of the population of the Madrid region live within 600 metres of a station and 2.4 million of them take a ride on the Metro each day. In peak periods, 333 trains are running at once.

Most of the rolling stock was built by local manufacturer CAF, with the exception of two classes made by Ansaldo Breda using Bombardier propulsion systems. There is also a three-line tram system called the Metro Ligero, using Alstom Citadis 302 stock.

Incidentally, there are several unusual features about the Madrid metro. Unlike the rest of Spanish railways, trains run on the left-hand track which has a gauge of 1445mm (4'87/8"). Power is collected overhead but a large part of the network uses a solid catenary system rather than conventional cables. The original 600V DC has been largely upgraded to 1.5kV.

Controlling the Metro

With a system of this size, its numerous control outposts became a logistical headache and had to be brought together. Central Traffic Control opened in 1983 and was followed three years later by centralised electrical supply, security and station control. However, these were in different locations making communication difficult. So, on 25th July 2000, a new Central Command Post was opened at the Alto de Arenal Station. In a room of 500m2, a team of over 20 controllers supervise train traffic, energy, passenger information, stations and safety. There is even a resident policeman to coordinate any law enforcement action.

The sheer scale of the system they are controlling is impressive. Rafael Lopez, Head of the Central Control Post, explained. "In addition to the trains themselves, there are 115 electrical substations, 1,475 ticket machines, 1,716 turnstiles, 4,285 CCTV cameras, 4,775 PA loudspeakers, 2,349 video screens, 1,594 escalators, 38 travelators and 483 lifts!" It seems that expansion has continued!

Despite the size of the network and the sophisticated control systems, even more capacity is needed. As Metro de Madrid doesn't want to lengthen platforms - they are all underground and extension of the caverns would cause enormous disruption - the only solution is to run more trains more frequently. And that means the signalling system needs to be upgraded.

To put this challenge in some context, Line 6 is a circle line, 23km long with 27 stations. The current signalling system allows Metro de Madrid to run a maximum of 21 trains in one direction and 18 on the other. The minimum interval between them is 178 seconds. By 2011, capacity will have to increase to 29 trains in each direction and a maximum interval of 100 seconds.

So in 2004 the decision was taken to improve the signalling on Line 6, together with Line 1 which runs north-south through the city centre. It is 23km long with 33 stations and carries 110 million passengers a year. Bombardier was asked to install its CITYFLO 650 system on both lines. This involves CBTC (Communications-Based Train Control) which had already been proven in service in several metros in the USA.

CBTC

This is how it works. Using a two-way radio system with a leaky-feeder coax in the tunnels, a train sends a signal to the central control centre giving its exact location. That information is then passed back, in real time, to the following train which uses the information to create a moving block safety zone, calculating how far behind it is and whether it needs to slow down or speed up. This second train is also providing positional updates to the control centre so that information can be relayed to the next train in line, and so on. All the calculations are made on the train, not in the control centre.

As always seems to be the case, things aren't quite as simple when it comes to implementation. Firstly, the lead train has to know where it is. This is a train-based system so there are no external axle-counters or track sensors. Instead the train calculates how far it has moved from a known point using tachometers on the axles. At each station, it reads a fixed transducer which it uses to correct its position so it can once again start from a known point. Transducers, mounted along the track, are totally self-contained and have a battery life of seven years.

Advantages

Of course the obvious advantage of all this is operational - trains can run closer together and more frequently, giving greater capacity on the line. However, selecting the Bombardier CITYFLO 650 system brought other benefits as well.

As there is very little trackside equipment, installation of the leaky-feeder cables and the location transducers took place during normal night-time maintenance schedules. The cables are pinned to the side of the platforms and tunnels. With the transducers powered by battery, there is almost no need for routine wayside upkeep.

Similarly the trains themselves were fitted with the necessary equipment during regular maintenance. Apart from a new visual display on the driver's panel, there is little sign of the newly-installed computing power.

Complications

The trains were already running under a track circuit-based ATP system. This had to continue in operation while the new CBTC system was installed. Existing interlockings were replaced with Bombardier's EBI Lock 950.

Passengers also had to be informed. Metro de Madrid launched a public campaign to highlight the benefits of increased capacity and allay any concerns that reduced intervals could have a negative impact on safety. In fact the new control system identifies all the other things that can affect service levels such as sticking doors and rolling stock malfunction.

In operation

The first passenger-carrying CBTC trains ran in July 2008 on Line 6 and in May 2009 on Line 1. Both are now fully operational in CBTC and the legacy ATP can be dismantled. Metro de Madrid has experienced an increase in capacity greater than they expected and is very pleased with the new Bombardier CITYFLO 650 system.

There is the capability of moving to full driverless operation (UTO - Unmanned Train Operation) as this is built into the system although Metro de Madrid has no plans to go down that route for the time being. In the current STO (Semi-automatic Train Operation) mode, the driver is responsible for opening and closing the doors and launching the train. He can also switch off the CBTC system and revert to full manual operation using conventional fixed-block interlockings as a fallback.

"The close cooperation between Metro de Madrid and Bombardier staff no doubt contributed to the success of this project" believes Joe Bastone, Bombardier's Director of Business Development. "We have a well-integrated team and every aspect of the project has been worked on together. We are very proud that we have installed a completely new control system on two lines at once without having to close the metro down at any time."

Now the operators have to get used to the new system and all the benefits that it brings. After that, there is always Line 10 and Line 3...

Article courtesy of the rail engineer magazine.

Quart squeezed into pint pot

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