Complex sentence: The'D' type cars were the least common of the car types: of the 66 cars built with guards compartments, only 18 were fitted with control equipment and coded'D' in 1923.
Split into simpler sentences: The'D' type cars were the least common of the car types. Of the 66 cars built with a guard's compartment, only 18 were fitted with control equipment and coded'D' in 1923.