There is now additional strike action on Saturdays 15th, 22nd and 29th September.
https://www.northernrailway.co.uk/news/latest-news/2009-northern-responds-to-rmt-s-september-strike-announcement-2
https://www.rmt.org.uk/news/strike-saturday-and-further-dates-on-northern-rail-announced/
Ticket acceptance with TPE will be in place, so if for example you have an advance purchase ticket to Sheffield via Brigg, you may travel by appropriate alternative trains.
Fee free refunds are available for anybody who chooses not to travel.
Further RMT industrial action on Northern now planned for Saturdays 6th, 13th and 20th October.
http://www.rmt.org.uk/news/rmt-announces-further-strike-action-on-arriva-rail-north200918/
https://www.northernrailway.co.uk/news/latest-news/2043-northern-responds-to-rmt-announcement-of-three-further-days-of-strike-action
https://www.northernrailway.co.uk/strike
And again for 27th October, 3rd and 10th November
https://www.rmt.org.uk/news/rmt-confirms-raft-of-new-strike-dates-on-northern-rail/
https://www.northernrailway.co.uk/news/latest-news/2066-northern-responds-to-new-rmt-strike-announcement
and for every Saturday in January:
https://www.northernrailway.co.uk/strike
https://www.rmt.org.uk/news/rmt-confirms-raft-of-new-strike-dates-on-northern-rail191218/
As usual, all Barton line services should run, but people at Cleethorpes, Grimsby and Habrough note that the Brigg line trains will not run.
https://www.rmt.org.uk/news/rmt-suspends-northern-strike-action/
Industrial action now suspended, it seems agreement has been reached to have a guard on every train.