braking arrangements was replaced by two characters as follows: An additional digit denoted the class of passenger accommodation: The final suffix indicated the build of the coach: This resulted in a 4 character alphanumeric TOPS classification, such as AB21 representing a Mark 1 Brake Corridor Second (BSK). See also Coaches of the London, Midland and Scottish Railway References Further reading External links www.departmentals.com British Rail numbering and classification systems Big four British railway companies