Train Tracks

Train Tracks


This mission is an adaptation of the "Tracks" game (from Simon Tatham's Portable Puzzle Collection ). If you are lost or just want to play, the game is available here .

You have to trace train tracks from "start" to "end" in a rectangular grid, respecting constraints:

  • Each row and column must contain a given number of tracks.
  • Some tracks are fixed and you must use them as they are. Their positions are given with the entry / exit directions (North / South / West / East). You can choose the entry.
  • The exit direction of the start, and the entry direction of the end are fixed.
  • The track may not cross itself.
You will have to return how you move from start to end with North / South / West / East directions.

  • Rows (and columns) count constraints will be represented by a list of integers.
  • Start (and end) will be represented by its coordinates, a tuple of two integers.
  • Fixed tracks will be represented by a dictionary with coordinates (a tuple of two integers) as keys and a set of entry / exit directions as values.
Note: (len(rows), len(columns)) give the grid dimensions.

Input: Two lists of integers, two tuples of two integers and a dictionary (keys: a tuple of two integers; values: a set of one or two strings).

Output: A string.


8x8 example image
Your moves from (3, 0) to (7, 6): "NNNESESEENENESSSESSWWSWWSEEE".
train_tracks([4, 6, 5, 3, 1, 3, 3, 4],  # rows
             [4, 2, 2, 3, 4, 5, 6, 3],  # columns
             (3, 0),                    # start
             (7, 6),                    # end
             {(3, 0): {'N'},            # constraints
              (4, 7): {'N', 'S'},
              (6, 4): {'E', 'W'},
              (7, 6): {'W'}},

To play the puzzles / tests yourself: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17


  • All puzzles are solvable.
  • 8 ≤ len(rows) ≤ 50 and 8 ≤ len(columns) ≤ 50.
  • all(0 ≤ row < len(rows) and 0 ≤ col < len(columns) for row, col in constraints)
  • all(dirs ≤ {'E', 'N', 'S', 'W'} and 1 ≤ len(dirs) ≤ 2 for dirs in constraints.values())
  • {coords for coords, dirs in constraints.items() if len(dirs) == 1} == {start, end}

Invalid hot key. Each hot key should be unique and valid
Hot keys:
CheckiO Extensions

CheckiO Extensions allow you to use local files to solve missions. More info in a blog post.

In order to install CheckiO client you'll need installed Python (version at least 3.8)

Install CheckiO Client first:

pip3 install checkio_client

Configure your tool

checkio --domain=py config --key=

Sync solutions into your local folder

checkio sync

(in beta testing) Launch local server so your browser can use it and sync solution between local file end extension on the fly. (doesn't work for safari)

checkio serv -d

Alternatevly, you can install Chrome extension or FF addon

checkio install-plugin
checkio install-plugin --ff
checkio install-plugin --chromium

Read more here about other functionality that the checkio client provides. Feel free to submit an issue in case of any difficulties.

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