Specialist police officers attended the scene alongside livestock handlers, and a drone was deployed to assist with locating and monitoring the bull.
However, it could not be safely contained or directed away from the live railway, according to a police spokesperson.
Insp Rob Campbell said the situation had been “exceptionally complex” and he understood the concern it might have caused.
“Our duty is to protect life and keep people safe. Together with British Transport Police, Network Rail and livestock handlers, we tried to contain and move the animal away from danger,” he said.
“Unfortunately, the bull continued to pose a serious and immediate risk on the railway.
“As a last resort, and with the appropriate legal authority in place, specialist officers humanely dispatched the animal to prevent any wider risk to the public.”