It's actually quite simple, when the running times were changed to 170 back in May it resulted in a small reduction in running time. Unfortunately this reduction isn't achievable, so therefore trains end up late. The structure of the timetable means that delays, even with the reduced timetable, can snowball. Trains pass just south of Bulwell South Junction and because of the additional stop in each direction (normally on the Mansfield train) the turnaround at Worksop is short. Therefore once a train ends up late going out it will come back late and delay a train going towards Worksop a couple hours later. Of course because of the unachievable running times now, a late train will loose further time. The reduction for an all stopping train is 4 minutes (2.5 out and 1.5 back) per return trip to Worksop, the reduction is all South of Mansfield and over 50% is on the single line section.