Those Vaia cranes are actually used for a whole host of duties.
They are, occassionally, used for brush cutting. But there's better equipment out there for that. Their primary duties is just picking up stuff. They have a small variety of different attachments that can be used on the end of the arm. They're probably most often used for picking up ties, but can be used to pick up virtually anything an MOW gang would need to pick up except ribbon rail, which is generally Grove crane duty.