For safety reasons, Disney requires that all strollers be folded prior to boarding the bus or the parking tram.
On the Monorail, only double strollers need to be folded to fit into a regular section. (They will fit into the wheelchair accessible car, however, guests travelling with mobility devices get 1st priority) All single strollers can roll right onto the monorail.
Most boats also allow for the stroller to remain unfolded, however, some of the smaller crafts will require the stroller to be folded prior to boarding.
One thing we do recommend... please have your stroller already folded, and all your "gear" in hand, when you reach the front of the line. It makes boarding so much easier and quicker when you've already organized everything.