TRENTON, N.J. (AP) — Amtrak is slowing trains along the busy Northeast Corridor because the heat wave is driving up internal temperatures of the rails.

READ MORE: Preparations Underway For The Salute To America Independence Day Parade In Philadelphia

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, Pa., and Philadelphia.

READ MORE: Suspect Arrested Following Fatal Stabbing In Wissahickon: Philadelphia Police

Amtrak says speed restrictions are implemented for safety when internal rail temperatures exceed 120 degrees. Amtrak engineers monitor track temperatures in real-time.

The railroad says the heat loosened overhead power lines on the Northeast Corridor and that caused delays between Metropark and Trenton, N.J., on Wednesday afternoon.

(© Copyright 2013 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.)

MORE NEWS: Chick-Fil-A Earns Top Spot For Customer Satisfaction In America For Eighth Straight Year: Report