Prefabrication techniques allow Ms. Bauer’s crews to close individual lanes instead of shutting down the bridge. Since February, they have torn out and installed one length of deck each night, and they have already completed a third of the task, she said.

None of the techniques is quite as eye-popping as “heavy lift” — when a hunk of bridge is simply picked up and put into place.

Time and the elements had not been kind to the steel and concrete of the old River Street Bridge, which stretches over railroad tracks used by freight and commuter trains. The bridge also needed raising — an additional 18 inches would allow double-decker commuter trains to pass underneath.

So the Massachusetts Department of Transportation got to work.

It had upgraded its own inspection and replacement processes after the August 2007 collapse of an Interstate bridge in Minneapolis, said Richard A. Davey, the Massachusetts secretary of transportation. It put its focus on rapid replacement, which tends to cost the same as slower approaches, if not less.

“The highway department didn’t use to see the drivers as customers,” said Frank DePaola, administrator of the highway division for the department. “For a while there, the highway department was so focused on construction and road projects, it’s almost as if the contractors became their customers.”

One local resident who is happy about the quick work is Gov. Deval Patrick. “It’s their money, after all,” he said. “And it’s their broken bridge.”

At River Street, workers started on the project last year, and began building the new superstructure on an adjacent lot in recent months. On Friday, the department shut down the rail line, leveled the track area with gravel and covered the tracks with sheets of plywood and steel to accommodate the trailers. On Friday night, heavy machinery tore out the old bridge, and on Saturday workers installed precast concrete caps on the old bridge abutments, shaped to accept the new, higher superstructure.