Yes, it is in the CBA.
From the CBA:
(page 23)
"A newly-hired employee without previous mechanical experience shall be classified as a Probationary Apprentice and shall work as a probationary employee for a period or periods totaling twelve (12) months within the aggregate period of not more than eighteen (18) months. The Company and the Union shall have the privilege of testing the ability of Probationary Apprentices during this twelve (12) month period. If they agree that the Apprentice during this probationary period does not display sufficient aptitude to become a first year Apprentice he/she shall be discharged at any time during the probationary period as stated above."

However, do note that it says "...without previous mechanical experience..." I think it is very unlikely that a person without any previous mechanical experience would get through the interview process.