Gilliam says the path that led to Tuesday's Oakland-San Francisco round trip started as serendipity: After seeing a Netflix documentary on dogs earlier this year, she started thinking about working with service animals.

Several days later she saw an online post from a friend who runs a horse rescue ranch in Monterey County. The friend was in the process of bringing more than 30 miniature horses seized in a Texas hoarding case to California.

Gilliam, who had run her own equine and canine rescue program near San Luis Obispo before moving to the Bay Area, knew that miniature horses qualified as service animals under the Americans With Disabilities Act. That history and the sudden appearance of a herd of tiny horses at her friend's ranch gave her an idea.

"I thought, 'Hey, miniature horses are ADA compliant, so let's go pick some out and give it a whirl,' " Gilliam said. She wound up adopting two minis, as the small horses are often called: a 9-year-old chocolate brown gelding she named Cuzzi and a 4-year-old pinto mare she called Sweets.

She started training the horses in March, a difficult process for animals coming from a situation in which they had been neglected.

Cuzzi — the name rhymes with Suzie — was sociable from the beginning, Gilliam says. Sweets "took a lot longer to warm up."

"We would go in her stall, and she would run to the back of it, and we had to work on, 'Hey, you can trust me, I'm OK, I'm not going to hurt you,' " she said.

Gilliam says Cuzzi is probably unsuited to become a service animal. He's friendly enough, but he tends to be skittish in unfamiliar situations. For instance, when Gilliam and a friend took the horses on an outing.

"We tried taking them to the movies — 'The Lion King,' which is, ironically, really kind of hilarious," she said. "But his nerves were too much, so we stayed for two previews and then left. You don't know how they're going to act until you take them to that situation. You prepare them as best you can at home by throwing everything you can at them."

How exactly did Cuzzi exhibit nerves?

"Lots of defecating," Gilliam said. "Every boom and light flash, he got startled."

By contrast, Sweets was so calm at the theater that she appeared ready to fall asleep. Gilliam says that's one of the horse's characteristics that made it plausible to her take on a more challenging adventure.

"Service ponies need to be able to go into heavily populated areas, ride on public transportation and aid people," Gilliam said. She describes Sweets' work role, when she's ready to take it on, as being "a mobility support animal — trained to help people getting around."

Miniature horses appear to have debuted as service animals in the United States about 20 years ago. That's when a North Carolina horse trainer and a visually impaired Maine client made headlines when they took a miniature horse named Cuddles on a trip that involved airline flights and riding Atlanta's MARTA transit system.

The client in that case, Dan Shaw, said one of the reasons he thought he'd prefer a mini horse to a dog as a service animal was its lifespan.

"Horses live 35 to 40 years," he told the Los Angeles Times. "I'm an animal lover. To lose a dog after eight to 10 years, and then have another to train, and have to do that three or four times in my lifetime ... that’s painful."

Gilliam says horses have a vision advantage over dogs, too. Their eyes, positioned on the sides of their heads, give them a 350-degree field of vision.

She believes equines are fundamentally different from canines in another important way, too.

"They are prey animals, where dogs are predators," she said. "That means they are going to be more alert to potential dangers."

Armed with those qualities, and eminently intelligent and trainable, miniature horses now appear more often as service animals — on planes, on trains and in restaurants.

And now on BART. Gilliam says that Tuesday trip had a couple of goals.

"We had just bought a new set of tennis shoes for her, and we were just testing out her maneuverability in her new sneakers," she said.

But there was a bigger aim, for both horse and trainer, than trying out new shoes.

"The whole experience was about trust," Gilliam said. "How much does she trust us? How comfortable is she? How can we manage her stress? How can we expose her to things and make it not stressful next time?"

How did the journey unfold?

Gilliam says she and a friend drove the 200-pound Sweets in a minivan from her barn near Fairfield to MacArthur BART in Oakland. Unable to find parking there after the morning rush hour, they drove to the Rockridge Station and parked nearby.

From there, they brought Sweets up to the fare gate and then to the platform on the station elevators. They immediately drew the attention of both BART employees and patrons, whom Gilliam says were overwhelmingly curious and friendly.

Leg one of the trip was from Rockridge to Montgomery Station in downtown San Francisco. After doing an errand there, Gilliam, Sweets and friend strolled down Market Street to BART's Embarcadero Station for the ride back to Oakland. Back at Rockridge, Sweets tried something new, leaving the station via the down escalator.

The trip wasn't problem-free. Under federal ADA regulations, the No. 1 requirement for miniature horses working as service animals is that they be housebroken. And Sweets didn't pass that test Tuesday, defecating on both train trips and peeing while riding on a BART elevator.

"Putting her in those stressful situations, she did have a couple accidents," Gilliam said of the onboard episodes. "But we had a poop catcher, and anything that missed the poop catcher we immediately cleaned up and sanitized." She added that an attendant on the elevator waved off her offer to deal with that mess, too.

Correcting that shortcoming in Sweets' travel manners "is definitely the next thing we will be focusing on," Gilliam said, adding that the horse is an excellent student.

"She's so smart and has such a good work ethic and is so willing to just do anything to please us, it's amazing," she said.

With more work to do, Gilliam says she has not placed Sweets with a client — yet. But she says mini horses have a future as service animals.

"I think these animals are incredibly smart and adaptable to being useful service animals, so I think people are going to be seeing more," she said. "And I’m sure there are a lot of people out there who want to see Sweets out on BART as 'the BART pony' again."