Metro-North said a scheduled bridge opening of the Cos Cob bridge in Greenwich could cause some train delays Friday morning.

The opening is planned for 8 a.m

“This opening could cause potential delays of 20 to 25 minutes. We will provide updates as needed. Please listen for announcements at your station,” Metro-North.

The movable bridge of the Mianus River was built in 1904. It is the busiest moveable bridge on the New Haven Line, which carries nearly 40 million passengers annually.

There have been delays in the past when maintenance crews had difficulty in securing the locking mechanism after the drawbridge closed.

The four largest rail bridges in Fairfield County -- the Cos Cob Bridge in Greenwich, the Saugatuck River Bridge in Westport, the Norwalk Walk Bridge and the Devon Bridge over the Housatonic River are all more than a century old.

Only Norwalk’s Walk Bridge, built in 1896, is being replaced.