4 ms·

Melbourne's new Metro tunnel (and Cranbourne, Pakenham and Sunbury lines) will use high capacity moving block signalling, with a minimum headway of 120 seconds!

• throwawaythekey 5y ago
  > [1]: https://bigbuild.vic.gov.au/projects/metro-tunnel/about/technology/high-capacity-signalling https://bigbuild.vic.gov.au/projects/metro-tunnel/about/tech... This video was great thanks! My naive assumption was that as trains operate in a relatively controlled environment a headway much smaller than 120s would've been easy to achieve. Cars regularly drive sub one second from each other! Is it just about minimizing the chance of catastrophe or is there more to it?
  • alfiedotwtf 5y ago
    Even though Melbourne trains are slow, I'd assume you'd want a long stopping distance and time for a train
  • jakecopp 5y ago
    Car stopping distances from 60mph/100kph are ~100 metres. 39,888 people died in the US in 2018 from car crashes. [1] Emergency braking distance of a TGV from 300 kmph (186 mph) is something like 3,500 m [2], carrying ~450 people. Moving block signalling systems take into account the current positions and velocities of both trains, and generate a maximum speed and braking curve. See [3] for the details, it's pretty fascinating systems design! [1]: https://en.wikipedia.org/wiki/List_of_countries_by_traffic-related_death_rate https://en.wikipedia.org/wiki/List_of_countries_by_traffic-r... [2]: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/16334/1991_SAFETY%20RELEVANT%20OBSERVATIONS%20ON%20THE%20TGV%20HIGH%20SPEED.PDF https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/16... [3]: https://en.wikipedia.org/wiki/European_Train_Control_System https://en.wikipedia.org/wiki/European_Train_Control_System
  • iggldiggl 5y ago
    Cars are driven based on relative braking distances, i.e. it is assumed that the vehicle in front of you will never come to a sudden stop. So you only need to keep a certain smaller margin to account for your reaction time (and some drivers ignore even that). This mostly works, but sometimes that assumption breaks down and you get something like a mass pile-up for example. Trains are held to higher safety standards, and so even modern train control system are usually based on absolute braking distance (and even when you want to space trains closer together regardless, as soon you need to throw a set of points between trains at a junction you're back to absolute braking distances, because as long as they aren't set safely towards one direction or the other, a set of points is effectively a stationary obstacle), i.e. there's always enough free space in front of the train to safely brake to a stop. This limits how closely trains can follow each other, and on urban railways it's normally the combination of dwell times (doors opening, passengers alighting and boarding, doors closing) and platform reoccupation times (wheels starting turning on the first train leaving the station to wheels stopping turning on the next train arriving at the station) that determine your minimum headway. (And for actually usable – as opposed to merely theoretical – capacity you also need to add at least a small amount of extra margin to compensate for possible dwell time variations and other day-to-day occurrences)
  • bluGill 5y ago
    While it is true that cars regularly are less than a second apart, safety experts give 2 seconds as the minimum safe distance, and 3 is often recommended. You should be the different person who follows their advice, no matter how many people cut in front of you. Better late than dead, and highways are very dangerous.
• fy20 5y ago
  That's the same as on the London Underground, in 2011 they rolled out 30 trains per hour on the Jubilee line and a few years later on the Northern line. The Jubilee line is 36km long, and each train is 125m [1] so that means there is on average just over a kilometer between each train at 30 trains per hour. That might sound like a lot, but stopping distances of trains are much greater than cars. First of all the trains weigh around 200 tons plus another 70 tons for passengers, and metal on metal doesn't have as much traction as car tires. Even if you could decelerate as fast as a car, everyone standing up is going to end up in a pretty bad shape after that so you need to do it slower. According to someone on StackOverflow passenger trains are designed to decelerate at 1.2m/s2 [2], which at 100km/h gives a stopping distance of 350m. [1] https://en.wikipedia.org/wiki/London_Underground_1996_Stock https://en.wikipedia.org/wiki/London_Underground_1996_Stock [2] https://www.quora.com/How-long-does-it-take-for-a-train-to-come-to-a-complete-emergency-stop https://www.quora.com/How-long-does-it-take-for-a-train-to-c...
  • lmm 5y ago
    The Victoria line is up to 36 trains per hour at peak times now. (And the Moscow metro runs 60 trains per hour, though some have questioned whether their practices would meet western safety standards).
    • iggldiggl 5y ago
      Can you point to a source for the 60 tph in Moscow? I know Moscow/Russia/ex Soviet Union is always mentioned in that context, but the figures usually given are more in the 40 – 45 tph range.
  • iggldiggl 5y ago
    The pinch point limiting capacity on urban railways normally isn't the plain line between stations, though, but rather the stations themselves: It's the combination of dwell times (doors opening, passengers alighting and boarding, doors closing) and platform reoccupation time that determines available capacity.