jingsmonty
Member
It’s often known as ‘marginal time’. Essentially it is the use of time in diagrams at the beginning and/or end of the day that would otherwise be unused to extend services beyond their normal point. The Chieftain is a classic case: when it was first introduced it used a set that would otherwise finish at Edinburgh at around 1630 to extend to Inverness. Likewise the southbound makes productive use of a set that would otherwise be sat at Craigentinny until 1100ish.
Explained far better than I tried to!