An engine kept at a depot must exchange two cars: car A at station A and car B at station B, on a track...

logic/train.shunting New

New entry, edited from material collected for the archive and not yet in a released version.

Problem

An engine kept at a depot must exchange two cars: car A at station A
and car B at station B, on a track that includes a bridge and a depot
siding.  What if the bridge is too narrow for the coaches but
wide enough for the engine?

[ Editor: problem statement reconstructed from the postings.  Only the
  variant question (asked by Michael Chew <m.chew@ic.ac.uk>) and its
  solution survive; the original "railway track puzzle" and its track
  diagram are not in the collected postings, so the layout given here
  is inferred from the solution. ]

Show solution

Move the engine to station A, grab car A and stick it on the depot
siding.  Go back through station A, across the bridge to station B and
push car B just past the siding junction.  Back up and go down the
siding to get A.  Back up off the siding, then connect A to B.  Push the
AB combo onto the siding, decouple B, pull A back to station B,
decouple A.  Back over the bridge and around to the siding.  Grab B,
back out of the siding, then push it into station A.  Now go back to
the depot.

-- Dean Edmonds <deane@io.org>