
Symmetry Minute - unixhero
https://en.wikipedia.org/wiki/Symmetry_minute
======
niftich
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...](https://en.wikipedia.org/w/index.php?title=Symmetry_minute&oldid=865986472)

~~~
itcrowd
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?

~~~
wongarsu
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.

------
Smaug123
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?"

~~~
Y_Y
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.

~~~
boomskats
You mean, like those bananas?

~~~
H8crilA
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...](https://en.m.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A9tat)

------
oneplane
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.

~~~
unixhero
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.

------
p0cc
Similar Submission (1 day ago) about Denmark's Train ambitions:
[https://news.ycombinator.com/item?id=20464602](https://news.ycombinator.com/item?id=20464602)

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

