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You go up the Grand Central line towards Bradford Interchange, before diverting outside Knottingley, onto the Dearne Valley line, before up to York via Sherburn-in-Elmlet and Church Fenton :P
Out of interest, is the route via Gainsborough LR and Lincoln still retained for diversions? I remember many years ago just after privatisation it was used frequently, but haven't seen it used for some time.
Wouldn't that be the northern half of the Joint line diversions? If so it must still be quite common, often 4-5 times a year there are diversions between Peterborough and the North via Sleaford and Lincoln.