Logic puzzle with rails: lay one track from the entrance to the exit so that every row and column holds exactly the number of track cells shown. Timed run from 6×6 to 10×10.
Drag your finger from cell to cell to lay track between them; a drag that starts along track you already laid erases it instead. Tap an empty cell to mark it ✕ (no track), or tap a cell with your track to clear it. The ✕ Mark button lets you paint ✕ over several cells with one drag, and Undo takes back your last move.
The track enters at the green arrow on the left edge and leaves at the yellow arrow on the bottom edge. It must be one continuous line made of straights and quarter-turn curves: no branches, no crossings, no loops and no stray pieces. The number above each column and to the right of each row says how many cells in that line hold track. A number turns green when the line is right and red when it has too much. Pieces on the blue-grey squares with a dot in the corner are given and cannot be changed.
Practice 4×4 is a short untimed lesson. A run starts with three 6×6 puzzles, then three each at 7×7, 8×8 and 9×9, and every puzzle from the 13th on is 10×10. Each puzzle has its own clock: 90, 125, 160, 200 or 240 seconds by size, minus 5 seconds for every puzzle you have already played in the run, with no lower limit. The clock stops the moment the route is complete, while the little train runs it. When a clock runs out the run is over. Your score is the number of puzzles solved, and ties are split by the spare time you had left, added up to a tenth of a second. Puzzle 49 would have no time at all, so a run can never go past 48 puzzles.
Start with the lines that are already decided. A row or column whose number is 0 is all ✕, and a line whose number equals the cells still open in it is all track. Once a line holds as many track cells as its number, every other cell in it is ✕; mark them straight away, because those ✕ marks are what make the next deductions visible. The same counting works the other way round: if a column needs 5 and only 5 of its cells are not crossed out, all 5 are track.
Then follow the ends. Every track cell has exactly two connections, so a track cell with only two open sides has to use both, and a cell with at most one open side can hold no track at all. That is why corners and cells squeezed between ✕ marks or the outer wall fill in so fast. Keep extending the loose ends of every piece you have, starting from the given pieces and the entrance and exit arrows.
Two rules about the whole route settle the last few cells. The track can never close on itself, so two loose ends of the same stretch of track must not be joined. And the stretch coming from the entrance may only meet the stretch coming from the exit once every number is already satisfied, since joining them finishes the route; if a link would connect them early, that link is a ✕ edge and the track must take the other way.
Every puzzle is built so that these deductions alone finish it, with exactly one answer, so you never need to guess. When you feel stuck, look for a line that is one cell short, or an end with only one way left to go. In a run, speed matters too: lay long straight stretches in one drag, use ✕ Mark mode to sweep finished lines, and lean on Undo rather than clearing the board.