PARIS — For more than 150 years, the bridge over the Little Androscoggin River in South Paris has provided a rail link between Portland and Montreal. At one point, it was determined to be so important that soldiers were stationed to protect it from sabotage.

This year marks the 100th anniversary of the bridge’s construction, even though there has been a bridge at the site since 1850, said John Davis, a local railroad history buff.

Davis said that when the railroad was first constructed, an investor from Paris Hill put lots of money into the project but stipulated that a train had to reach South Paris by Jan. 1, 1850.

However, the first bridge was still under construction as the deadline approached. With some quick thinking, Davis said, engineers rigged the bridge temporarily to allow an engine to cross, meeting the investor’s requirement.

“Under railroad terms, even just the engine by itself constituted a train,” Davis said. “Therefore, on Jan. 1, they were there with a train.”

The bridge was completed in February 1951, when the first train — with cars — rolled into the South Paris station.

According to Davis, the bridge burned in 1872 and was replaced with a steel structure in 1876. It was rebuilt again with a steel span between 1900 and 1901.

Over the next 12 years, heavier engines began to expand the stone abutments, so the company rebuilt the bridge with a steel trestle. It was completed in 1913.

When World War I broke out in 1914, there were fears that the Canadian-owned Grand Trunk Railroad, including the Little Androscoggin bridge, would be targeted by saboteurs.

According to a Maine History Online article by historian Candace Kanes, these fears were heightened when a German infiltrator detonated a bomb at the Vanceboro bridge on the Maine-New Brunswick border.

Even though the U.S. remained neutral until 1917, soldiers from the 2nd Maine Regiment were posted to guard bridges along the rail line, including the bridge in South Paris, Davis said. When the U.S. entered the war, the 2nd Maine returned to regular service, leaving responsibility for the bridge to hired civilian guards.

Davis said there was at least one episode when guards fired shots at what they said was a man, even though no evidence of a suspect was ever uncovered.

The Grand Trunk was nationalized by the Canadian government in the early 1920s and the Portland-Montreal line saw reduced traffic.

Davis said not too much went on with the bridge until the U.S. entered World War II in 1941, when protecting the line again become important.

According to Kanes’ article, African-American troops from Fort Devens, Mass., were dispatched to guard a variety of strategic rail sites across the state.

Although Davis said he hadn’t found any reference to troops stationed in South Paris during the war, he’s convinced there must have been a unit to guard the bridge.

In the past half-century, there has been a reduction in rail traffic, and passenger services have constricted. Only freight comes across the bridge now. Gone are the days when a traveler could get on the train in South Paris and arrive in Chicago 62 hours later, without switching cars, Davis said.

But for those, like Davis, who love the idea of train travel, there might be a light at the end of the tunnel. More people are discussing the idea of resumed passenger service to Western Maine and many proponents see the revival of passenger service as a way to boost tourism.

One day the bridge in South Paris might again carry people, as well as freight, across the Little Androscoggin.