Today was the big day! The first time she pulled the cart with someone in it. And it was rather uneventful. Another boarder sat and watched with me, she and the trainer were joking that we had front row seats to the big rodeo. Afterwards she said she wanted a refund, LOL. Of course, the trainer and I expected her to act exactly like she did: chilled out and unphased. The trainer even had her do some short trots. Normally he will NOT ask a horse to trot the first time with a driver in the cart, but well... Sierra is not average. Her trots were extremely half-hearted, though. She wasn't too fond of the idea of carrying the extra weight around, me thinks! As long as she "trots" when asked, he won't ask any more out of her for the next 6-12 times he drives her. He just wants her to get comfortable, then he'll working on making her move out like she needs to.
She was completely overwhelmed, clearly.