The rounded shapes of the hoods and cab gave it a baby-brother appearance to the main line 42 and 44 classes.

The addition of a train brake allowed them to be run on the main line and also to be used as replacements for various ageing shunting locomotives.

Their engines were more powerful rated at 290 hp (220 kW) at 2,100 rpm and they had a higher top speed of 32 mph.

The exterior design of the Mark 2 was squarer, giving it the appearance of a baby-brother to the 49 class diesel locomotives.

The second six being an additional order, were placed into traffic from May 1968, with the final member of the class entering service in December 1968.

largely successful, as they began to replace the various ageing steam shunting locomotives in various depots of the time.

They were cleaner, more efficient to run, requiring less maintenance, and also giving locomotive crews more comfortable working conditions.