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 L3.

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 L3 2-6-6Ts to the Lima Locomotive Works for rebuilding in 1928 through 1930. At this time their classification was changed to D2a to avoid conflict with the class L Mohawk types of the New York Central System. The rebuilt engines had 23x24-inch cylinders and a boiler pressure of 200 pounds, and exerted a tractive force of 34,260 pounds. Their 63-inch drivers, small for locomotives in passenger service, helped them get their trains moving more quickly after their frequent station stops. They weighed 172,700 pounds. The rebuilt No. 306 is shown here in a view taken in Boston on July 12, 1933 by Roy F. Blackstone of Eldon, Missouri.