How to play Train Tracks

Train Tracks is a one-line logic puzzle: no guessing is needed, because every board has exactly one solution that the numbers pin down.

The rules

1Find the entrance and the exit
Two dark tabs sit on the edge of the grid. The arrow on the first points in, the arrow on the second points out. Your track must run from one to the other.
2Read the numbers
Each number says how many squares in that row or column contain track. Turn a number green by hitting it exactly; go over and the board won't accept the piece.
3Draw one unbroken line
Straight pieces and corners join end to end. There are no branches, no loops and no crossings, and no piece can have an open end.
4Check the grey squares
Squares shaded grey already hold a piece. They're free clues, so build outward from them.

Draw: Drag along the route. A square that continues the line lays the right piece by itself; touch a square that isn't part of the route and you lose a life.
Pick a piece: Choose a straight or a corner from the tray and tap a square to place exactly that piece.
Mark and erase: The ✕ marks a square as empty; the eraser clears a square.
Undo and hints: Undo takes back your last move. Practice puzzles have two hints each; the Daily has none, so everyone plays it the same way.
Lives: Easy gives you 5, Medium 4 and Hard 3. Run out and you can try the board again.

Zeros first
A 0 means the whole row or column is empty. Mark it with an ✕ so you can see what's left.
Count what's left
If a clue is 4 and only 4 squares in that line aren't crossed out, every one of them is track.
Work from the tabs
The line has to leave the entrance square straight away. The clue for that row or column often tells you which way it turns.
Corners tell you more than straights
A line that turns uses squares in two rows and two columns at once, which is usually where the numbers finally fit together.
Use the ✕ tool
Crossing off squares you know are empty makes the remaining possibilities obvious, and it costs nothing.