They had Hank on the monorail, that's where Astor was born.So probably their current enclosure by the monorail is not bull proof if you're referring to a much older habitat.
In 1981, the facility welcomed the only Asian elephant ever born in New York. “We had four elephants who grew up at the Bronx Zoo but the male only showed an interest in breeding with one female,” Doherty mentioned.
Research and deductive reasoning are your best friend, don't take everything at face value.
https://www.zoophoria.net/single-po...herty-former-general-curator-of-the-bronx-zoo