From Trinidad South either wait for the internal or walk to Jamaica and hop on a Studios, Epcot or Animal Kingdom bus.
It would be shorter to just walk from Trinidad South to Old Port Royale versus walking from Trinidad South to the Jamaica bus stop and then from the OPR bus stop to the building.
I thought this off the top of my head, but then I used my favorite running website to actually get the walking distance measurements down to one-onehundreth of a mile.
Starting from the identical spot in Trinidad South ...
... if you walk along the Promenade past Trinidad North make a right through the pool complex (where the baby pool and the locker room/bathroom building used to be), you get to the side door of the gift shop. This walk is 0.38 miles.
... if you walk along the Promenade to the roadway and walk along the roadway to the Jamaica bus stop (if it is even safe for pedestrians!), it is 0.40 miles. Then from the OPR bus stop to the main back door to the building is 0.06 miles, for a total of 0.46 miles. If you can't walk along the roadway and would have to meander through Jamaica and the parking lot, then it would be a lengthier walk.
The walk to the Jamaica bus stop is not only slightly longer to begin with, but then, of course, you'd have to wait for the bus to drive you to the OPR bus stop.
Given this route data from satellite photos, it would make the best sense to just hoof it from Trinidad South to Old Port Royale as opposed to getting a park bus at Jamaica to take you to OPR.
Of course, the shortest walk would entail waiting for the internal shuttle to pick you up at Trinidad South, and then get off at OPR.
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