Many moons ago I managed a unit with 15 staff. It functioned well, not least because for 15 months we didn't lose a single day to illness. After the 12 month point I knew that record would soon fall and it did. In quick succession 2 key members were off for over 4 months each. Pressure came on the rest and we sometimes had 5 of the 15 off sick together, plus usual holidays, training and courses. Inevitably things didn't work quite so well!
Fully staffed is very difficult to define, and rather depends on the viewpoint it's looked from. 15 was the establishment. We could fully function with 12 present, but once allowances get made for holidays, training and sickness, especially when unplanned, it's different. All businesses are much the same. The level of cover that needs to be maintained to cover eventualities, and how it's done, can get silly.
Back in the 60s national employers recruited heavily in the North-East where unemployment was high, and slowly shunted staff towards London. Hostels were even provided for them in the capital. Relief staff got generous expenses packages to fill gaps across the nation. The jobs I knew weren't too highly skilled.
Members of the public find it hard to understand railway conditions where a driver can't just sit in the cab and drive away, any train, any route, any time.
Given the conditions that do apply on railways it is very difficult to say that Northern is under, or over, staffed. It will be both at the same time, in different places, in different grades, and even on different days of the week. That's where effective management is critical. Changing ownership in itself hardly matters without full understanding of the nuts and bolts of the business. How and why we got here, where we are, then where and how we're going next.
Once the new trains are all in place, the cascades are all in and refurbished, and with old stock removed we'll be better able to judge performance. It might be 6-12 months before things start to settle down. How the May 2020 timetable works out will be the big test. We need to be realistic.