In simplistic terms operational planners, whether in a TOC/FOC or at Network Rail, spend most of their time amending the base plan.
A rough approximation is that 25% of the work is doing the permanent timetable, and 75% is then amending it for Bank Holidays, Engineering Work, Special Events and short notice freight.
Generally the more junior staff tend to learn their trade on Short Term Planning, which gives them the experience needed to work on the base plan when the opportunities arise, assuming they haven't left by then
