PORTLAND — The prolonged heat threatens to put Amtrak’s Downeaster in the slow lane.

The Northern New England Passenger Rail Authority is warning of delays along the Downeaster route because of speed restrictions caused by the heat.

Search photos available for purchase: Photo Store →

The heat causes the steel rail to expand, and speeds must be lowered for safety.

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

For the Downeaster, there can be speed reductions down to 40 mph from Maine to the New Hampshire border. That means extra travel time between Portland and Boston.

Amtrak is also slowing trains elsewhere along its busy Northeast Corridor

Passengers traveling between Boston and Washington on Amtrak and commuter trains could expect delays of up to 20 minutes Thursday.

The speed restrictions also were in effect on portions of the Keystone Corridor between Harrisburg, Pa., and Philadelphia.

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.