Normal sudoku rules apply. A greater-than symbol points towards the smaller number. Draw two metro lines that travel orthogonally from cell to cell, never branching, crossing, or overlapping, either with themselves or with each other, which eventually close to form two loops. Thick black walls cannot be crossed by the lines. One loop must lie entirely inside the other, so the outer loop completely surrounds the inner loop. Every station must lie on a loop. All cells on the same loop share the same parity (the two loops may have the same or different parities). A digit in a red sensor equals the number of cells visited by either loop in the 3×3 box centred on that cell. A digit in a station equals the total number of stations containing that digit. The mirror cell has mirrored parity, meaning its parity is considered to be the opposite of its real parity. If a loop passes through it, treat the cell according to this mirrored parity.