Interestingly enough South West Trains and now South Western Railway would talk about running additional services to the Saturday timetable over the Christmas holidays.But then that doesn’t cover where trains are short formed or long-formed, as many operators are doing. By that measure Cross Country are at around 60%. By vehicle miles they are at around 90%.
But if you go to vehicle miles, TPE and actually over 100%. c2c were at something like 140% over the summer.
No easy answer.
As far as I could tell, on the Poermsourh line at least, these would just be existing services extended further down the line. So maybe starting at Petersfield instead of Haslemere. I can't remeber the exact examples so I've made one up. You get the idea though.
I'd always think of additional services in terms of additional trains but that isn't always the case.