All were built in American Locomotive Company's plant at Schenectady, New York. The first nine built in 1910 (builders numbers 47732-47736 & 49207-49210) were 5,000 pounds (2,300 kg) lighter than the infobox figure. Sub-class W-1 consisted of seven locomotives added in 1912 (builders numbers 50933-50939) and eight more (builders numbers 52989-52996) delivered in 1913. The last four designated sub-class W-2 (builders numbers 54564-54567) were delivered in 1914 and were 4,000 pounds (1,800 kg) heavier than the infobox figure.[1]
Class W locomotives were numbered from 501 to 528 as delivered. Numbers 501 and 519 were officially property of the European and North American Railway (E&NA) as a condition of the lease of that company by the Maine Central Railroad. They therefore avoided scrapping until Maine Central purchased E&NA to remove the lease obligation in 1955. Two locomotives survived; No. 501 is currently under restoration to operating condition at the Conway Scenic Railroad and No. 519 was on display outside at the Steamtown National Historic Site, exposed to the elements.[2][3]
When the Maine Central began purchasing diesel locomotives, road switchers were numbered in the 500 series previously reserved for the W class.[4]ALCO RS-2s and ALCO RS-3s were numbered 551 through 557, and EMD GP7s were numbered 561 through 569 and 571 through 581.[5]