The federal Department of Transportation on Monday awarded Amtrak the final funding it needed to replace the 116-year-old railroad bridge that carries trains over the Connecticut River between Old Lyme and Old Saybrook.

Construction is expected to begin in 2024 and take five years to complete. The project is estimated to cost $957 million and the existing bridge eventually will be demolished.

The $826.6 million award announced Monday is part of what President Joe Biden described in a tweet as the largest investment in passenger rail since the creation of Amtrak: A total of $16.4 billion for 25 passenger rail improvement projects along the Northeast Corridor between Boston and Washington, D.C., the busiest passenger rail corridor in the country.

It is one of nine projects that will upgrade railways in the state. The projects are funded by the 2021 Bipartisan Infrastructure Law.

U.S. Rep. Joe Courtney, D-2nd District, said in a press release that the grant announced Monday would allow Amtrak to complete construction of the new bridge without stopping to acquire additional grants. He said he has been working with his colleagues to secure the funding since 2007.

The 1,570-foot-long Connecticut River Bridge, built in 1907, carries an average of 38 Amtrak trains, 12 Shore Line East and six Providence & Worcester freight trains a day. However, it has been subject to frequent delays due to rail and maritime traffic.

“This long overdue transformational grant for the Connecticut River Bridge comes after determination in 2006 that the bridge was ‘structurally deficient’ and repair work was no longer capable of keeping it functional,” Courtney said in the release.

In an interview Monday, Courtney said that aging bridges pose a greater risk for train operators, who have to slow down and be careful. The newer, more structurally sound bridge would allow for higher speeds, he said.

The new bridge will be built to the south of the existing one, and will improve the reliability of train service and increase the authorized speeds for trains from 45 to 70 mph, according to a February presentation from Amtrak.

Courtney said that when he unveiled the news to a crowd in Old Saybrook Sunday, the group had burst into applause.

“This has been talked about for so long that I think people had gotten skeptical of if we were ever going to see the day,” Courtney said.

He added that he was excited to be able to complete a project of this magnitude.

Courtney said once the new bridge is complete, train traffic will switch to the new bridge. He said it would be difficult to say yet whether construction would disrupt rail or maritime traffic during the five-year construction period, but that the goal is to minimize interruptions and finish construction as quickly as possible.

“Every time you can upgrade this aging infrastructure, it just means that the goal of high-speed rail gets closer and closer. And that will in my opinion stimulate more ridership. Because for a lot of people it’s quicker, or in their minds it’s quicker to get in their car and drive,” Courtney said.

“The extent that we can speed things up safely I think is going to make passenger rail much more attractive,” he added.

d.drainville@theday.com