"You got your royal engineer's certificate! Now you're a full-fledged engineer!"
The Engineer Certificate, more formally known as the Royal Engineer's Certificate, is awarded to Link by Princess Zelda during a ceremony in Hyrule Castle in Spirit Tracks. The primary purpose of the Engineer Certificate is to certify an engineer of being capable of conducting a train. Additionally, this certificate also certifies that the awarded engineer becomes a Royal Engineer.
Link getting the Engineer Certificate.