Flippy the Train

Build a train track to solve digital circuitry puzzles! Flippy the Train rides the rails, flipping switches to model basic digital circuitry: NOT, AND, and OR logic gates. After building these basics, reuse them in larger tracks to create the more-complex XOR gate and half-adder. What is the fewest number of track segments required to build a half-adder? Includes a sandbox mode for creating arbitrary track for Flippy to ride.