A tank engine is a locomotive that carries its water and fuel supply on the same frame as the boiler and running gear, eliminating the need for a separate tender. Tank engines, also called "double-enders," are more easily operated in reverse than those with tenders. Several North American railroads used them in suburban service, where a full schedule of trips each day required a quick turnaround of their trains. In 1906, the Boston & Albany received an order of 2-6-6T tank engines from the American Locomotive Company, the rear six-wheel truck supporting the coal bunker and water tank. No. 306 was a member of this class L-3.

When Alco delivered a group of larger and more powerful class D1a 4-6-4T double-enders in 1928, the B&A sent the older L-3 2-6-6Ts to the Lima Locomotive Works for rebuilding in 1928 through 1930. In 1940 they were reclassified to D-2a to avoid conflict with the new class L-3 Mohawk types of the New York Central. As rebuilt, the 2-6-6T double-enders had 23x24-inch cylinders and a boiler pressure of 200 pounds. They weighed 172,700 pounds and developed 26,800 pounds of tractive effort. Their 63-inch drivers, small for locomotives in passenger service, helped them get their trains moving more quickly after their frequent station stops. The rebuilt No. 306, which was retired in 1950, is shown here in a view taken in Boston on July 12, 1933 by Roy F. Blackburn of Eldon, Missouri. Ron Morse of Springfield, Missouri, is the current owner of this photograph.