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.