Author: Vladimir Portugalov
Draw a single continuous loop which passes horizontally or vertically through the centers of the cells. The path crosses itself at the marked intersections, and only at these crosses. It visits every other square exactly once. The train stations (indicated by numbers) must be passed through in strict numerical order. The path is not allowed to turn at a station, but must continue straight through the station's square.
Grid control: To draw a line press the mouse button at a start cell and move mouse to another cell holding the button. More details about grid control are at the help page.