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.

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

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.