As you and Bletchleyite have pointed out, this is imperative. Why is it not being done ? Are Northern somehow considered compliant with DfT's diktats if they pretend to operate the new timetable, even if in practice it does not work ? Or do they lack the capacity to draw up an emergency timetable ?
An “emergency” timetable I understand is being devised but it can’t be done overnight especially when you have to allocate train crew (which they are already short of) to those trains.
Quite likely the latter, their planning team is not big and they will be doing their best, but there is only a finite resource.
Playing the devil's advocate somewhat here, but having been forced into planning timetables in weeks rather than months due to the mess of engineering overruns / delays, Northern can hardly be expected to devise an emergency timetable in days rather than weeks. Yes they have had timetables in place for strike days, but these were probably long in planning and they have had time to tweak these as the dispute has gone along.
Yes they should be doing a better job, but they can't just announce a totally different timetable overnight. This month's changes haven't just been affecting Northern, TPE are having a 'mare and no doubt this is translating into VTWC, East Midlands et al. Its an unholy mess in the North West without a doubt, tempered somewhat by the upcoming Liverpool blockade which I'm sure will be used to take a long hard look at what has happened & try to formulate a solution. This isn't the first time that planning has gone up the spout, nor are Northern the first company to get things wrong (look at the mess that was Southern for a very long time). All we can all hope for is that the said blockade gives them time to work out a better plan, and for the DfT to ask serious questions of all TOCs concerned, Network Rail & its contractors as to why things are just so bad.
Edit: Just a supplemental thought here, this is exactly the kind of leverage the train crews in dispute with Northern could seek to use. I wonder if anyone in the unions concerned has thought of offering a deal to end it by offering the goodwill of the crews to work some extra shifts etc? Given the current growing bad press, Northern / Arriva would be remiss to ignore any love branch offered to help end at least some of the chaos in return for some / most demands of the dispute met? As an ex-union rep this is the sort of thing I'd be working on, however I'm an ex-union rep because too many of my fellow reps where stuck in the 1970's and so this kind of thinking was beyond then, so maybe I'm being a bit over-optimistic!!