The problem was the Sheffield-Gainsborough/Lincoln line ran west-east and crossed the north-south EC main line on the level just south of Retford station. So trains from Sheffield to Gainsborough needing to stop at Retford would take a spur to the north of the station, then a sharp right hand curve round into the station. They would then leave the station by a sharp curve to the left at the south end and rejoin their line.
This imposed severe restrictions on the EC main line, and in the 70s an underpass was built [leaving Newark the sole surviving flat crossing]. New platforms were built in the underpass to allow Sheffield-Gainsborough/Lincoln trains to stop at Retford independently of the EC trains. But the spur from Sheffield round to the north-end of the EC main line station was retained, and there was even a service from Sheffield to London Kings Cross and return which used it.
That spur was eventually singled, and what you see in the video is the infrequent Northern service which terminates in Retford's down platform on the EC line, rather than the Gainsborough platform in the underpass.
