although the dd was considered to be a successful design, it had a key shortcoming in that its boiler performance was not sufficient for the traffic demands being placed on it. in 1914, an experimental superheater was fitted to d 882 and was found to be very successful. both d and a designs (both locomotive classes still under construction at the time) were modified with superheated boilers (with all of the existing a class locomotives eventually fitted with superheated boilers). superheaters were also fitted to three of the formula_1 locomotives. further d locomotives were also built with 19 in. diameter cylinders in place of the original 18 in. cylinders.