How to mix the traffic outside and inside the heavy density network

By Cally / 17 June 2013 / 0 comments

More often than not, the traffic influx in big cities will differ drastically from one area of the network to another. Providing a blanket signalling approach across the whole network is neither cost-effective nor necessarily sufficient. Signalling Consultant Carlos Rico Gomez, our guest correspondent at MetroRail 2013, describes how to optimize different sections of the network to deal with different volumes of traffic.

In the modern cities there is a flow of people travelling from the outskirts of the city to the city centre for business or leisure. This fact provokes a scenario where there are different needs in the control and signalling of the different areas in the network.

Most of the cities have a very dense section of the track, in some cases with different lines converging into the same areas, even into the same platforms, while in other sections the density is low and headways are big.

Clear examples of this are:

Configuration in Y, like Metro Bilbao, Salvador de Bahia, and many applications in central Europe.
Configuration in X, like Metro Dusseldorf, Metro Caracas and Metro Brussels.
Example of the Metro Bilbao Network.

Dealing with such a diverse degree of needs is not easy. The most logical approach is to let the more powerful solution drive the complete network, but this is not cost efficient. Centralized control traffic will require a strong Passenger Information System to help the travelers standing on the platform understand the waiting time, and the possible different combinations in order to optimize the travel time.

The main problem is the signalling: with the same system we have to drive 4-6 minutes headway in the outskirts and 2-3 minutes headway in the city centre. There are few systems flexible enough to do both scenarios.

Most of the administrations will choose CBTC for the complete network, however this penalizes the cost in the outskirts. Others would pick ERTMS, cheaper overall, but with difficulties in getting the necessary performance in the city center, the most critical part.

If the required headway in the outskirts is longer than 2 minutes, the most efficient way will probably be ERTMS L1 in the outer city, with big track sections (800-1000 m), and then go to ERTMS level 2 in the densest part with short track section (200-400 m).

Trying to do everything with ERTMS level 1 will not be very operative because of the infield information. On the other hand, doing everything with ERTMS level 2 will bring unnecessary costs and GSM-r coverage

If the required headway in the densest part is 90 seconds or less, ERTMS will not be able to provide the necessary functions. Therefore CBTC will have to be implemented in the whole line, even if the investment in the outskirts is above the strictly necessary.

Different suppliers cover the range between 90-150 seconds headway. They have different systems that can provide compromises more favourable than ERTMS L1 and CBTC. This will work if there is a big difference of requirements in the different sections of the line.

What will solve this problem will probably be ERTMS level 3, because it will most likely be compatible with level 1&2, and will also include moving block capacities. In that case ERTMS Level 1 can be used in a very easy and cost effective manner in the outskirts and ERTMS level 3, with moving block, would be used in the city centre. However, ERTMS level 3 is still in a specification phase, so it will still require a little bit of time.