@najaB is bang on: the original requirement that this batch of 73s were rebuilt to meet was to cover the Highlander portions solo - including Inverness. In the real world it was found that they didn't offer the expected performance and that, in particular, reliability suffered if running very hard for extended periods.
Hence the general practice appears to have been adopted to prioritise shed assistance on the Inverness (heaviest) followed by Fort William and finally Aberdeen (least demanding).
Sometimes a second 73 has been used in place of the 66 but again the shed option appears to be preferred.