A miniature horse drew big smiles on one of San Francisco's Bay Area Rapid Transit (BART) trains this week.

Employees allowed the service animal to board the train Tuesday after its owner provided the proper paperwork, BART said in a statement.

"This mini horse is a service animal, and the owner provided paperwork to a station agent stating so," BART said. "After consultation (station agents usually don’t see horses), the horse was allowed in."

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"Trained service animals assisting people [with] disabilities and on [a] leash are welcome on BART," the transportation system added.

A Twitter user posted a photo of the white-and-brown equine Tuesday with the caption, "retweet BART pony for good luck."

The post had more than 3,600 retweets and more than 5,000 likes as of Wednesday night.

The horse quickly drew attention and became the highlight of some passengers' day.

"BART had a major delay tonight but I would do it all over again if I could see this pony," wrote Kaitlyn Moore.

Another passenger said it "made my day!"

"So cuuuttttteeee! Is it bring your miniature horse on BART day?" they asked.

The Justice Department revised the Americans with Disabilities Act (ADA) in 2010 to add a provision about miniature horses that are trained to perform tasks and assist people with disabilities. Service animals are defined as "dogs that are trained to do work or perform tasks for people with disabilities," the ADA said.

However, "entities covered by the ADA must modify their policies to permit miniature horses where reasonable," according to the regulation.

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An assessment of the service animal should include whether or not the horse is housebroken and under the owner's control, whether the facility can accommodate the horse, and whether or not the animal's presence will compromise safety requirements, the ADA said.