Symmetry Minute | Hacker News Reader

Symmetry Minute

en.wikipedia.org

84 pointsunixhero18 comments

Although I'm familiar with this concept since the implementation of ITF (integrated clock-face timetables) in Central Europe, I found the second half of this article (as of this revision [1]) hard to read. After finishing it, I felt like I understood less about clock-face scheduling than I did before.

Nonetheless, I know that the key desirable features are (1) to ensure that hub-like nodes have services from all directions arrive and leave at the same time, so that connecting passengers don't need to wait for long, and (2) that a future arrival/departure time at a major node is somewhat predictable without consulting the full timetable.

For trains, it's also a nice feature that trains of opposite directions will meet somewhere predictable, so track improvements like double-tracking can be targeted at places where crossings are likely to occur, which can cost less than double-tracking the entire line.

Integrated clock-face timetables work best when rigid, but it requires the spacetime of transport geography to fit into a regular pattern. But this can mean that some improvements that would result in faster service on portions of the network would put the the network out of sync. Because of this complication, such incremental improvements may not happen.

[1] https://en.wikipedia.org/w/index.php?title=Symmetry_minute&o...

I had the same problem with the Wiki article! But, to be honest I had the same with the last part of your comment. Could you explain your last paragraph in other words because I don't really understand what you mean there?

For simplicity's sake assume we have two short train lines, one takes one hour from start to finish, one takes two hours. They always meet at the same point, everything works out. But if you can improve the track to speed up line 1 by 10 minutes everything goes out of sync.

This article in German [1] and this source in Hungarian [2] demonstrate the concept with very clear charts you can likely understand without having to read the labels. The distance-time graphs show that different trains move at different speeds: faster trains have a steeper slope, slower trains have a gentler slope and more spots where they dwell for a few minutes before advancing.

You want your graph symmetric around various point-in-time axes, so that the scheduling is predictable.

On the distance-time graph, if you've upgraded a line and can run faster trains, you can plot the train with an even steeper slope in the graph. But by doing so, you may miss more of the knots where many different lines in the graph come together. These knots in the graph are moments in space and time when several trains are at the same station around the same time: the time to change trains and make connections.

The system works best when major hubs are roughly 1 hour apart, so you begin optimizing for that fact systemwide, instead of small, few-minute travel time reductions on individual lines. It may be the case that small improvements in travel time on one line don't outweigh the reduction in connection opportunities, so small improvements become harder to justify. You'd have to run certain trains much faster (and spend more money on faster tracks) to maintain the same level of connections in the network.

[1] https://www.zukunft-mobilitaet.net/42868/analyse/integraler-... [2] http://www.itf.hu/index.php/alapfogalmak/integralt-uetemes-m...

This is amusingly similar to the old chestnut "I have a rope of nonuniform thickness, and it will take one hour to burn its entire length. How can I measure half an hour?"

Burn it at both ends?

This is usually presented as having 2 ropes, not 1, and 45 minutes, not 30 minutes.

https://www.cartalk.com/radio/puzzler/string-series-number-1

I don't think this non-uniform, flammable rope would be allowed in the EU. Even then it's hardly the right tool for the job of measuring time.

You mean, like those bananas?

Hey it's not just the EU that is strangely obsessed with bananas. It's a big deal m'kay, governments were toppled and CIA was involved:

https://en.m.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A...

Burn it at both ends at once?

Reminds me of this simple analog computing device: ^_^

https://www.exploratorium.edu/snacks/center-gravity

> Here’s an easy way to find the center of gravity of a long, thin object, even if the object’s weight is unevenly distributed.

Weigh it?

To me, the most interesting thing about this is that I was familiar with the implementation but never figured this was a thing on its own or even has a name. Makes sense that stuff like timing, scheduling etc. has a field of knowledge on its own. This makes me wonder what other unknowns in the area of commonly used processes I don't know about.

Actually. These incredibly interesting Wikipedia articles pop up here on HN from time to time. And with that here is my contribution. I suggest you search for wikipedia.org with the HN search engine to find more Wikipedia articles with interesting stuff.

Actually. These incredibly interesting Wikipedia articles pop up here on HN from time to time.

Similar Submission (1 day ago) about Denmark's Train ambitions: https://news.ycombinator.com/item?id=20464602

Top comment is related and references Clock-face scheduling (https://en.wikipedia.org/wiki/Clock-face_scheduling).

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