Amtrak is slowing trains along the busy Northeast Corridor because the heat wave is driving up internal temperatures of the rails.
Passengers traveling between Boston and Washington on Amtrak and commuter trains can expect delays of up to 20 minutes Thursday.
The speed restrictions also are in effect on portions of the Keystone Corridor between Harrisburg and Philadelphia.
Amtrak said speed restrictions are implemented for safety when internal rail temperatures exceed 120 degrees. Amtrak engineers monitor track temperatures in real-time.
The railroad said the heat loosened overhead power lines on the Northeast Corridor and that caused delays between Metropark and Trenton, N.J., on Wednesday afternoon.
Amtrak said it is also responding to the extreme heat by stocking additional water on board trains for passengers, strategically positioning extra crews and locomotives and providing additional safety briefings for employees about working in extreme heat.