In 1845, the Hartford and New Haven merged with the Hartford and Springfield Railroad to form the New Haven, Hartford and Springfield Railroad. The company reverted back to the Hartford and New Haven Railroad name in 1847.
On April 4, 1839, the Massachusetts legislature granted a charter for the Hartford and Springfield Railroad, which was authorized to build from the Connecticut border to Springfield. The new railroad company never operated independently, as it was simply a vehicle for the Hartford and New Haven to extend its line into Massachusetts. Construction began in 1842, and the first trains between Hartford and Springfield ran at the end of 1844. The complete route was 62 miles (100 km) in length.