Derek was not talking about a Mansfield to Newark line via Rainworth, Farnsfield and Southwell. That line was closed and tracks lifted years ago and much of that line at the Mansfield end is now built upon and doesn't now even connect with the current Robin Hood line through Mansfield.
What Derek was meaning was the line that leaves the Robin Hood Line at Shirebrook and runs through Warsop, Ollerton and Tuxford to Lincoln. Much of this line at the Nottinghamshire end is still fully intact.
I thought there were two Derek's replying to this thread, when I saw that!!

Actually a lot of what remains of the former LDECR is in as good condition as many a main line, since the High Output Track Renewal Train was commissioned on the test track, & work as been on-going along the branch for some time now. Clipstone East Junction was plain-lined in the summer & the West junction is to also due to be plain-lined as well. There is some work being done to upgrade the signalling, so maybe Clipstone West box will be closed in the near future, although I have heard that some of the semaphores will stay.
I doubt that Network Rail will want to give up the section used for the test track at this stage, but at a future date maybe they would. I have heard talk of the possibility of a new cross-over at Thoresby, keeping one line for the test track & the other for passenger trains, as far as Ollerton, where a single platform station would be built.
I doubt that they would get the funding to reintroduce passenger services at the moment but I'm getting more convinced that it will happen at some stage in the future. So it would start looking more attractive to re-lay the missing link.