Metro

1. Every numbered stop has a twin somewhere on the map. Drag from one to the other to lay that line.
2. Lines run between neighbouring cells, straight or around a corner. Two lines never share a cell.
3. The network is finished when every cell carries a line. There is exactly one way to do it.

Drag over a cell another line already uses and that line is cut back to make room. Tap a stop to start its line over.