Photo by Kevin H.

Just how bad is this heat wave getting? It’s forcing Amtrak to slow its trains along the Northeast Corridor between Washington and Boston, as intense temperatures are causing the train service to implement additional safety protocols.

The Northeast and Mid-Atlantic are in the fourth day of a brutal heat wave, with temperatures from D.C. to New York in the high 90s and matching humidity creating heat indices of 105 degrees or more.

Amtrak says it implements speed restrictions when rail temperatures reach or exceed 120 degrees. As a result, trains along the Northeast Corridor and Keystone Service (between Philadelphia and Harrisburg, Pa.) are running about 20 minutes behind schedule.

Ordinarily, regular Amtrak trains running between New York and Boston reach top speeds of about 135 miles per hour, while Acela trains hit about 150 miles per hour.