The railway 's Operating Department had to use mismatched classes of underpowered and obsolete 4 - 4 - 0 and 0 - 6 - 0 locomotives .
Operating Department could run within the restrictions imposed by the infrastructure .
The railway 's Operating Department had to use mismatched classes of underpowered and obsolete 4 - 4 - 0 and 0 - 6 - 0 locomotives because they could run within the restrictions imposed by the infrastructure .