Boston & Albany class D-1a

These heavy tank locomotives were designed for commuter service in the Boston area. Thanks to the tank locomotive design rarely used in the USA, they could run in both directions at high speed. A three-axle rear bogie was used to carry sufficiently large supplies. This led to the 4-6-6T wheel arrangement, which had previously only been used on smaller Mason Bogie locomotives.

In normal operation they often hauled ten steel cars and were apparently able to move up to 20 of these cars with sufficient acceleration. They were regarded as successful all-purpose locomotives, but were only used in the Boston area due to their small number. They bore the markings of the New York Central for most of their lives. When they were replaced by diesel locomotives in their traditional area of service around 1950, no new purpose was found, so they were probably scrapped subsequently.