We took our Maclaren Triumph last week. For magical express, the driver stowed it underneath for us.
The Triumph folds pretty fast. When we would see our bus at the stop, we would go in high gear trying to rush to make it. My husband would grab our son and the tote bag. Then I'd fold up the stroller real quick. Just one time, we were holding up the bus a bit while scurrying to get our son out of the stroller. I started to bring the stroller onboard & fold it up once we were on board. The driver barked at me to fold it up before I got on.
We left the stroller in the stroller parking areas outside each attraction and never had a problem. It was always there where we left it.
I was surprised that we couldn't take it in Imagination, Living Seas, or The Land at Epcot. Made for a lot of carrying. We used the puppy backpack leash some too.