McIntosh developed six different classes of 4-6-0 for the Caledonian Railway: • large
49 and 903 Classes for express passenger traffic, with 6' 6" driving wheels • intermediate
908 and
179 Classes for mixed traffic, with 5' 9" driving wheels • small (5') wheeled
55 and 918 Classes for the Oban line and express goods traffic respectively The 55 class had been introduced in 1902, specifically for use on the Caledonian's Oban line, and the 918 class was a development of the 55 class which was intended for express goods traffic on the main line. Whereas the 55s featured a relatively small boiler to keep axle weights low and very short tenders to keep within the length of the Oban shed turntable, the otherwise similar 918s had larger boilers and tenders. Although built for freight service they were also used occasionally on passenger trains, and like all Caledonian 4-6-0s they were painted in the Railway's blue passenger livery. ==LMS ownership==