I still stand by my earlier comment that it's a better connection or through service between the Robin Hood Line and Sheffield that is desperately needed and would be far better used than an extension to Retford.
Passengers from Nottingham, with lots of luggage, travelling to the North East and Scotland via the ECML are not going to want to travel for almost 1 hour 15 min on a local stopping service via Mansfield and Worksop to reach a very infrequent ECML service at Retford which always requires an additional change at either Doncaster or York. That is without thinking about the connection times at Retford.
Whenever, I travel on the Robin Hood Line to and from Worksop, I can't help that notice how lightly loaded the trains are throughout the day between Mansfield and Worksop and I am surprised that the line can currently even justify an hourly service outside peak times. That is certainly not the case on the southern half of the line where most trains between Mansfield and Nottingham are quite busy even with a half hourly frequency.
Most of the communities along the line north of Mansfield (all in Derbyshire!) including Shirebrook, Langwith, Cresswell and Whitwell, look far more towards Sheffield than Nottingham for employment, shopping and leisure. People from those stations would naturally probably far more prefer to travel to/from Sheffield than Nottingham but that journey will never attract many passengers with a 40 minute wait for a connection at Worksop on the outward journey. I am convinced that if a journey time of under 1 hour could be achieved from Mansfield to Sheffield the northern half of the Robin Hood Line would see significant growth in passenger numbers. You only have to look at the large numbers of passengers travelling to and from Worksop to Sheffield in comparison to Nottingham.
Currently, if ever I want to travel to Sheffield and beyond to stations in Yorkshire or to the Manchester area, I drive to Alfreton and park cheaply there or occasionally to Chesterfield, although parking charges are high there. I only ever use the connection at Worksop if I'm going away for a few days and want to leave the car at home and even then using the bus to Alfreton is often more attractive.
Even though it should result in greatly increased passenger numbers from Mansfield northwards East Midlands Trains are not interested in trying to improve the connection situation at Worksop. I have written to them on a couple of occasions and the response I get is that the main purpose of the Robin Hood Line is access to and from Nottingham and that is their priority.
Any timetable changes on the Robin Hood Line are going to be very difficult to bring about whether it is for extensions of the service to Retford or Sheffield or even better connections at Worksop with Northern Services. EMT see the main purpose of the service as serving Nottingham and its onward connections. I always find that hard to believe whenever I return from a day out somewhere arriving in Nottingham in the evenings often with a long wait because of the badly timed connection.
The single line sections south of Kirkby in Ashfield will always cause problems in timetabling. Currently to speed trains up through this sections trains miss out the stops at either Newstead or Bulwell to allow the half hourly daytime frequency to run to time. Even a few minutes change to the timings in either direction will cause problems here. As both the Sheffield and Lincoln services just about pass at Worksop and arrive and depart at approximately the same time each hour it ought to be easy for good connections to be provided to all destinations. If the train from Nottingham arrived in Worksop about 7 minutes before the Northern Services and then departed again about 7 minutes later that would mean good connections and only a maximum of 15 minutes layover time for the EMT service. There are one or two times in the evening where this arrangement currently takes place. To do this it would perhaps need a slight speeding up of the Robin Hood Line Services by Worksop services missing out both Newstead and Bulwell. However, it would probably be very difficult to do this because of timings over the single line sections and platform occupation and movements at Nottingham.
I know that my posting has been rather long and complicated but I do use the Robin Hood Line regularly and know many of the problems.