Prior to around 1900 the Midland's locomotives were not noticeably different in size or power to those of other British railway companies; what was more notable was the company's commitment to standardisation of a small number of related locomotive designs. This policy began in the 1850s with Matthew Kirtley as Chief Mechanical Engineer. Kirtley provided two basic engine types - 0-6-0 locomotives for freight and 2-4-0 types for passenger work. Over 800 Kirtley 0-6-0s and 150 2-4-0s were built up to 1873. Samuel W. Johnson and Richard Deeley continued the policy but with a gradual progression in the locomotive designs. By 1914 the entire Midland network was being operated by six basic engine designs: a Class 1 0-6-0 tank engine for light freight and shunting, Class 1 0-4-4 tank engine for light passenger work, Class 2 4-4-0 engine for general passenger work, Class 3 0-6-0 engines in tank and tender variants for mixed traffic and freight, and Class 4 4-4-0 for express passenger work. This degree of standardisation was exceptional amongst the pre-grouping British railway companies. All the Midland designs were built to the same basic design principles and a 'kit' of parts meaning that many parts such as boilers, cylinders, wheels, cabs and bearings were interchangeable across some or all of the six types.