Groton Railroad Bridge

The Groton Railroad Bridge is a railroad crossing of the Thames River between New London and Groton, Connecticut. It carries Amtrak’s Northeast Corridor.

The nation’s rail network, a key driver of American industrialization, was largely built out between 1840 and 1880. Most early lines were short routes designed to tap the economic resources surrounding individual cities, but by the second quarter of the 19th century, cities in the northeast began building longer routes and consolidating smaller lines to connect them more closely.

The New York, New Haven & Hartford Railroad was formed through the consolidation of the Hartford & New Haven Railroad Company with the New York & New Haven Company, and the merged railroad was formally established on August 6, 1892. ¹ Such consolidations were common in late-19th-century railroading, creating both transportation empires and monopolies. The peak growth years for American railroads came in the early 1900s, with 1911 to 1919 marking the industry’s final major surge before a decline driven by past monopolistic excesses and growing competition from other modes of transportation.

By the close of the 19th century, the New York, New Haven & Hartford Railroad’s network stretched from New York to Boston, giving it effective control over rail traffic across southern New England and blocking major competition along its routes. ¹ The railroad then undertook a substantial building program, concentrated mostly between 1911 and 1919, to secure that position. Advances in construction technology by that point allowed for the large-scale movement of earth and rock, letting engineers raise bridges above streets and cross rivers where crossings had previously been impractical.

Movable Bridge Technology

A key feature of railroad construction during this period was the movable bridge, a structure that can shift position to allow river traffic to pass. ¹ Three basic types existed: the swing bridge, whose movable span rotates on a pivot pier; the bascule bridge, which in its modern form uses a counterweight to raise one end of the span; and the lift bridge, whose span is raised vertically between two towers.

Early movable railway bridges in the United States typically used the rim-bearing swing design, including a series of timber truss swing bridges built across Boston’s Charles River in the early 19th century. ¹ Many rim-bearing swing bridges were also built throughout the Mississippi Valley during the 1860s. The center-bearing swing bridge, an improvement over the rim-bearing type, was refined between the late 1880s and 1900, and its use was strongly promoted afterward by C.C. Schneider, a consulting engineer for the American Bridge Company. Modern bascule and lift bridges did not emerge until after 1890, once the electric motor had been refined and effective counterweight-balancing methods developed.

Along the East Coast, the many navigable rivers and inlets crossed by what is now the Northeast Corridor resulted in the construction of 15 movable rail bridges: nine bascule bridges, five swing bridges, and one vertical lift bridge. ¹ Swing bridges were generally favored on wide waterways with clearance on both sides, while narrower waterways, common in the northeast, called for bascule or lift designs. Bascule bridges are harder to maintain and cannot open to a full 90-degree angle, limiting clearance for tall vessels, but they open and close faster than swing bridges. Lift bridges share the same clearance limitation but require less maintenance than bascule spans.

The New York, New Haven & Hartford Railroad built four movable bridges along its Connecticut shoreline route, at Niantic, Shaw’s Cove, Groton, and Mystic, all within 16 miles of one another. ¹ Niantic and Groton were built as bascule bridges, Shaw’s Cove as a rim-bearing swing bridge, and Mystic as a center-bearing swing bridge. Each was engineered for its particular site and anticipated operating conditions, and each was prefabricated at the contractor’s plant before being assembled on-site by unskilled labor. Because the operating machinery for these bridges was never standardized, each one has unique mechanical components.

These four crossings reflect the state of movable bridge technology between 1907 and 1919. ¹ Because steamship lines dominated Long Island Sound traffic through the 19th century, the push to complete an all-rail shoreline route between New Haven and Boston developed relatively late, not being finished until 1889, even though two other routes, via Willimantic, Providence, and Boston, and via Springfield, Connecticut and Massachusetts, offered comparable rail mileage between the same points. As the older shoreline bridges deteriorated and proved unable to handle heavier, faster rail traffic, they were replaced with movable bridges representing the most current engineering practice of the era.

1889 Bridge

The Thames River crossing at Groton posed a serious engineering challenge. In the early 1880s, a bridge here was considered beyond the era’s engineering capability: the river measured about 1,400 feet wide at this point, with water 50 feet deep at the center and a rock bottom lying 100 to 180 feet below the surface. ¹

A bridge was nonetheless built at the site in 1889 using the best techniques available at the time, but the structure soon proved unstable. ¹ Shortly after it entered service, the west end’s Pier No. 1 began to settle, and by 1905 the bridge had shifted enough that it was deemed unsafe for locomotives weighing more than 157,000 pounds; fully loaded 100,000-pound cars were barred from crossing, hampering efficient train operations. By 1908, pier settlement had become severe enough that traffic was reduced to a single gauntleted track.

1919 Replacement

Given this history of instability, engineers gave considerable attention to foundation stability when planning a replacement bridge. ¹ James Rollins of the Boston firm Holbrook, Cabot & Rollins studied several alternatives before settling on rectangular open crib caissons for the new piers. Although this method was expensive and time-consuming, the railroad considered it worthwhile for the stability it provided. Foundation work began in April 1916 and was completed in August 1918. The piers were designed to accommodate four tracks, though only two tracks were ever built.

Design of the superstructure also required careful consideration. A swing bridge is generally used only where two channels of equal width are needed, and while it requires less power to operate, it involves more structure and weight. ¹ A bascule span, by contrast, requires less structure and is less costly to build. Because pier stability remained the primary concern, the lighter weight of a bascule superstructure was likely the deciding factor in the design choice. The superstructure was completed in 1919.

The resulting Groton Bridge, built in 1919 by the American Bridge Company, was a Strauss heel-trunnion bascule bridge, one of the earliest movable bridge types developed after the electric motor was refined and counterweight balancing techniques improved after 1890. ¹ The design was prepared by the New York, New Haven & Hartford Railroad’s own engineering department, led by Chief Engineer Edward Gagel, Construction Engineer I.D. Waterman, and Bridge Engineer W.H. Moore, with plans and specifications for the movable span furnished by the Strauss Bascule Bridge Company of Chicago. Consulting engineers on the project included E.A. McHenry, J.P. Snow, Gustav Lindenthal, and Ralph Modjeski. Holbrook, Cabot & Rollins built the substructure, while C.H. Norwood Contracting Electrical and Mechanical Engineering of Chicago installed the bridge’s air, electric, and gas engine equipment.

The Strauss bascule design, patented by Joseph B. Strauss and first completed in 1905, came in three variants: the vertical overhead counterweight type, the underneath counterweight type, and the heel trunnion type. ¹ Groton Bridge was the only heel-trunnion Strauss bascule bridge built along the Northeast Corridor rail line.

As completed, the bridge consisted of five double-track through-truss spans. ¹ From west to east, it included two Pratt truss spans measuring 185 feet and 330 feet, a 212-foot Warren truss bascule span, and two additional 300-foot Pratt truss spans, for a total length of 1,394 feet between the faces of the west and east abutments. The structure’s live-load-carrying members were made of structural silicon steel, and the bridge provided a 25-foot clearance above mean high water.

Mechanical Operation

When opened, the through truss span rotated about trunnions located at the bottom chord of the trusses. An overhead counterweight was linked to the moving span and a fixed tower at the rear floor break by a parallelogram arrangement of struts; as the moving leaf rose, the trunnion folded and the counterweight moved downward, keeping the bridge balanced throughout its operation. ¹ The drive gearing terminated at two pinions that engaged a rack attached to the operating struts, which were themselves connected by trunnions to the top chord of the trusses; as the pinions turned the rack and struts, the bridge rotated about the heel trunnion.

The operating machinery was housed between the fixed trusses on the fixed span. Four 82-horsepower, wound-rotor AC motors were geared together in simultaneous pairs to provide power, supplemented by an auxiliary 40-horsepower motor in the machinery house. ¹ Each motor was fitted with a solenoid brake for routine stopping, and each operating strut carried a pneumatic emergency brake. Below track level at the toe of the bridge, a lock linkage with its own motor drive and gear train secured the span, with a manual bevel-gear backup; mitre rails with alignment guides sat at both the heel and toe of the bridge, and electrically driven sliding rail locks secured both ends. Power for opening the bridge was requested by the bridge operator from the signal tower, with electrical switchgear and controls housed in the operator’s house and the lower level of the machinery house. The two-story operator’s house stood adjacent to the northbound track at the rear floor break.

Partial 2008 Replacement

The New York, New Haven & Hartford Railroad was absorbed into the Penn Central Railroad following that company’s 1969 merger. ¹ After Penn Central’s bankruptcy, ownership of the bridge and the surrounding Northeast Corridor infrastructure passed to the National Railroad Passenger Corporation, known as Amtrak, in 1976. Amtrak has owned and operated the bridge ever since.

The bascule main span was replaced by a vertical lift span in 2008; the structure today includes a Warren through-truss vertical lift and four Parker through-truss approaches.