"Although the industrial action begins at 1830 hours, this means that members due to start work on or after that time will not be coming in. Members already at work will work until the end of their shift. So someone who booked on at 1400 hour, for instance, will continue to work until their shift ends. This means that the action builds up slowly and there will be very little disruption in the first few hours. The main impact will come on Monday morning. Therefore the Barnsley and Cardiff fans should not experience too many problems getting back to the mainline stations."
It's further reported that a resolution could still happen on Thursday that ends the threat of strike action altogether.