I think they are temporaily replacing the horse drawn trolley as the track is partially closed off at the railroad station loop so the trolley can not turn around for now.
They use this carriage occasionally. Mostly when crowds are low. My sister and I rode on it in late January. You are right that it is out now due to trolley not being able to run. Rode on it the 12th of Sept.
I've never seen them there before. However, I'd love to go on a ride in one. My BF and I love doing horse drawn carriage rides. We'll definitly be doing one at POR next year on our trip!