Yes, it's the job of the conductor, although the engineer may assist. A replacement knuckle weighs about 70 lbs. If the break is deep, the conductor will usually secure the rear portion then identify the knuckle type ('E' or 'F') and have the engineer toss one on the ground. Then he'll close the angle cock on the attached portion, let it charge, then pull the rear with him riding up to the replacement knuckle. If it's on the portion he is holding on to, he'll replace it there, then ride back to the hook. If the break is on the car left standing, he'll put the knuckle somewhere on the rear car and ride back, stop short, and replace it before doubling back to the standing portion and lacing it up then knocking the hand brakes off.

It's not always possible to do it this way though and in that case, the conductor may have to walk up to the head-end and heft the knuckle back to the break. Or, he may "borrow" one off the rear car if it's closer and the correct type. Every situation is different. If it happens in the yard, there may be mechanical people who can perform the repair. But out on the road, it usually falls on the conductor.