The capacity of the subway line in case of an incident

The station factor

Image result for subwayNobody is perfect. But I can live with that. So can the subway. In a previous article, I wrote about the carry capacity of a subway line and the factors upon which it depends. One of them is the time spent in a station. It can go from 30 seconds ( deceleration + stop + acceleration) to a full day. In the last case, the capacity drops to zero. But should  it become zero ? Aren’t there  alternatives to traffic disruption ? Fortunately, they are and I will write about one such solution.

The problem

disabled-trainWhen a train stops in a station, the following train needs to  come to a halt, and so does the train before it. It is a chain reaction. One single train blocks the whole network. What’s worse, it can block the trains going the opposite way, too.Maybe not instantly, but in 30 minutes, the whole network is at stall.

Introspection

While it is true that the train stopped in a station should block all the following trains, it doesn’t need to be true.  It is a bit counter intuitive. After all, one should ask the following questions:

Q1: how do I ensure traffic continues in all the other stations?

Q2: how to carry passengers through  the station where the train is stopped ?

Q3: how to remove the problem from the equation ?

Traffic continuity

line-incidentThe network  is a linear set of stations. Let’s suppose the subway line has 26 stations A to Z. the incident happens at station I.

line-a-h-i-j-zThere should be no issues with  the segments A-H, respectively J-Z. Traffic should be smooth. There are two possibilities:

  • stations H and J become end stations, i.e. no traffic through station I        OR
  • traffic continues to flow through station I

marshal-solutionWe see that the two possibilities are exclusive. For the first case, passengers must find an alternative to travel between stations H and J.  It is just a problem of marshaling.

ipcs-solutionWe are more interested in the second solution, which allows a slightly degraded traffic to pass through station I. As we can see, the train switch the railway, going in what could be called as opposite direction on the other rail. The whole process is described  here.

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