PORTLAND (AP) – Amtrak’s Downeaster has been unable to make a full run from Portland to Boston because of structural problems with a bridge that passes over railroad tracks in Kennebunk.

Repairs were continuing on the highway overpass that buckled during an overhaul of the tracks underneath last week. The work sent debris tumbling onto the tracks below, making them impassable.

The Northern New England Passenger Rail Authority cobbled together a system in which riders took buses from Portland to Wells on Sunday and Monday. From there, they were able to take the Downeaster to Boston.

The rail authority, which operates the Downeaster, said the bus alternative would continue Tuesday and that regular Downeaster service was expected to resume within the “next day or so.”

The bridge problem occurred last week during rail repairs undertaken while the Downeaster was shut down because of security concerns related to the Democratic National Convention in Boston.

Workers were in the process of strengthening the railbed in Kennebunk when the Route 35 overpass buckled.

The bridge is being removed and a temporary replacement is expected to be in place in a week or two.