Why can a powerful locomotive still need a banking engine?

Why might locomotive 60103 have difficulty climbing a hill on a run to York?

Because the railway no longer had the banking engines that once supplied additional power on demanding gradients.

A powerful main-line locomotive can still need assistance on a demanding gradient. The reported difficulty came from the loss of banking engines, which once supplied that extra effort over steep sections. The performance of an individual locomotive therefore depended on a wider operating system rather than on its own power alone. The banking locomotive Big Bertha is given as an example of support equipment that should have been preserved because it represented part of the railway's working capability.