The last ten Pacifics that the Boston & Maine purchased were also the last Pacifics that Lima manufactured. Their task was regional traffic over distances of up to around 200 miles, and they were designed for an average speed of 70 mph or 113 km/h. The firebox contained three thermic syphons and arch tubes. The coffin feed water heater was concealed in the smokebox. Initially there was a skyline casing on the top of the boiler, which was later removed. The locomotives from the two production years 1934 and 1937 were designated P-4a and P-4b. They were given names that were chosen in a competition among kindergarten children and students. Only 3713, which is currently being overhauled in Steamtown, has been preserved.

The class R-1 Mountains were the last steam locomotives ordered by the Boston & Maine. They were among the heaviest Mountains and were built in four batches between 1935 and 1941. Initially five R-1a were built, in 1937 another five R-1b, in 1939 three R-1c and finally five R-1d. All locomotives were given names that were determined in a competition.

From the R-1b onwards, the spoke wheels on the driven axle were replaced with disc wheels. The R-1c was almost identical to the R-1b, only the R-1d underwent major changes. It now had disc wheels on all drivers and the Coffin feed water heater was replaced with a Worthington one. In addition, there was now a seven-axle tender instead of the six-axle one. Thirteen R-1s were sold to the B&O in 1947, the remaining five were scrapped in 1955 and 1956.

In 1928, the Boston & Maine procured their first 20 Berkshire-type locomotives from Lima. This meant a leading axle, four driving axles and a two-axle bogie under the firebox. Thanks to the bogie, the grate area could be exactly 100 square feet. After the 20 locomotives of the class T-1a, another five were ordered in the following year and classed T-1b. These were slightly heavier than the T-1a, which were later upgraded to the same level.

Because the adhesive weight was relatively low compared to the power of the engine, the maximum cutoff was set at 60 percent. The actually achieved starting tractive force therefore deviates from the result of the formula usually used, as this assumes a cutoff of 85 percent. An additional 12,000 pounds of traction came from a booster driving the rear axle of the bogie. A special feature of the locomotives was the Coffin feedwater heater, which lay in a semicircle above the smokebox.

The locomotives were unpopular in operation, as the bogie was often the cause of problems. Because the weight of the firebox rested on it, high forces acted on the boiler when running through curves. In addition, the bogie tended to derail when reversing.

In view of the problems, the Boston & Maine was able to sell ten engines to the Southern Pacific and seven to the Santa Fe during World War II. The Santa Fé gave the locomotives a rebuild that increased boiler pressure to 270 psi and used smaller diameter, longer stroke cylinders. These were able to prove themselves in service until 1954, while the others had already disappeared in 1949.

Parallel to the K-6 with a two-cylinder compound engine, the Boston & Maine had ALCO in Schenectady build 34 class K-5a and b locomotives with a simple engine. They still had Stephenson valve gear but piston valves. According to “Railway and Locomotive Engineering”, the dimensions of the heating surfaces were very well matched to the size of the cylinders. They were retired between 1926 and 1936. The locomotives later designated as K-5d were actually K-7s with superheaters. 

The Boston & Maine's first Pacifics were 82 members of the classes P-1 and P-2. They were characterized by 73 inch (1,854 mm) drivers and cylinders which were sized 22 by 28 inches. The P-1 consisted of twelve saturated locomotives built in 1910 by ALCO. Between 1911 and 1916, ALCO delivered 70 P-2 which were superheated and had a much larger firebox. 

In 1915, also all P-1 were superheated at the cost of much of the tube heating surface. Later additions added to many P-1 and P-2 included an Elesco or Worthington feed water heater. All in all, they were referred to as unexceptional locomotives which did their work without many problems, but also without noteworthy records. With a factor of adhesion of 4.5 for the P-1 and 5.0 for the P-2, it doesn't look like slipping was an issue.