Playing with model trains and calling it graph theory

Re:Playing with model trains and calling it graph theory by vibrio on Friday May 03, 2019 @10:13AM (#19818588)

Thanks for this, I've been obsessing about track layouts while playing trains with my kid, how to avoid getting stuck in permanent loops, having bi-drectional routes on all parts of the track etc. As I'm not a math person, I've not had a logical framework with which to think about it. my kid doesn't care but laying out tracks been a surprisingly fun mind game for me.

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Re:Playing with model trains and calling it graph theory by grimjack00 on Friday May 03, 2019 @11:01AM (#19819025)

You may want to search for "shunting puzzles", such as those here: https://www.transum.org/Software/Shunting/Puzzles.asp

Every time my model train interest rises, I consider setting up a small layout for puzzles like these.

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Re:Playing with model trains and calling it graph theory by m463 on Friday May 03, 2019 @08:09PM (#19823360)

I've wanted to make timesaver for quite some time now.

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Re:Playing with model trains and calling it graph theory by rhizome on Saturday May 04, 2019 @12:32AM (#19824459)

Those look fun, thanks for the tip!

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Re:Playing with model trains and calling it graph theory by astazangasta on Friday May 03, 2019 @10:41AM (#19818856)

Ditto. The ideal track layout should allow trains to go in either direction across every segment. Bonus for going under a bridge. I also haven't really worked out the graph theory (something involving the +/- orientation of junction splits) but the temptation is immanent.

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Re:Playing with model trains and calling it graph theory by jpfed on Friday May 03, 2019 @11:55PM (#19824344)

I also play trains with my kid, and I might be thinking about the same stuff. When I build a track, I think "given a position and direction, could I reach every other (position,direction) pair from here?". There's a certain minimum complexity required for a track to have that property, but I haven't really broken it down in a principled way.

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Re:Playing with model trains and calling it graph theory by d_j_b on Friday May 03, 2019 @12:47PM (#19820005)

I've been doing this too, with mine! A recent discovery is that if you keep all the Y pieces pointing in the same compass direction you seem to end up with a track which is always 100% traversable in either forward or reverse.

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Re:Playing with model trains and calling it graph theory by Someone on Friday May 03, 2019 @08:51PM (#19823537)

That can’t be true, can it?

Traversable doesn’t change if you bend parts of a track, and that can change the compass direction the switches point at.

For example, if part of the track containing a switch has an S shape, moving the switch from the center horizontal to the top or bottom one turns it by 180 degrees without affecting traversability.

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Re:Playing with model trains and calling it graph theory by travisjungroth on Friday May 03, 2019 @01:26PM (#19820334)

I know nothing about train tracks past my BRIO days, but a bit about graphs. Can you explain how a track would not be bidirectional except for a permanent loop?

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Re:Playing with model trains and calling it graph theory by desertrider12 on Friday May 03, 2019 @01:54PM (#19820633)

Bidirectional without backing up. To turn a train around, you need a closed loop at the end of the line or a triangle (wye).

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Re:Playing with model trains and calling it graph theory by hakfoo on Saturday May 04, 2019 @03:04PM (#19828506)

One of the things that makes it more interesting in practice is that most electric model trains* use a "one rail is hot, one is return" electrical design-- so adding a wye or reversing loop to the design is asking for all sorts of electrical gimmicry to prevent it from shorting, while still maintaining usability.

*classic Lionel and Marklin designs used a third conductor in the centre. Since the outer rails are wired together, reversing designs are trivial.

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