PORTLAND — Significant traffic backups are expected in Portland Wednesday and Thursday when materials are delivered for the Downeaster rail extension.

The delivery will impact traffic at each of the railroad crossings from Riverside Street to Congress Street, said city officials. They recommend that commuters seek alternative routes to avoid delays.

The materials will be delivered starting at about noon Wednesday at the Riverside Street crossing and continue until 6 p.m.

The following crossings and intersections will be impacted for about 30 minutes: Allen Avenue, Read and Walton streets; Forest Avenue at Woodfords Corner and Saunders, Woodford, Revere, Lincoln, Coyle, Ashmont, and Congress Street.

The delays are expected to continue into Thursday.

The expansion of Downeaster service to Freeport and Brunswick involves about 30 miles of track rehabilitation and upgrades. Construction should be complete by the fall of 2012.