I’m not so familiar with railway terminology. Does this mean 50 conductors working during a standard working day?
Here is a crude example of a railway link I've just made in Excel. This link has three lines.
A. Person would start on line 1 of the link and have Mon, Tue, Wed off (Rest days). Then they would work Thu,Fri,Sat,Sun and then go to line 2 in the link. J. Bloggs would start on line 2 and then the week after go to line 3. Once D. French finishes their first week on line 3 they would go back to line 1. It's a rolling rota system.
The more lines in a link the more work the depot has and the more staff it needs. The link requires one staff member per line in the link to make it work. So you can conclude that Newcastle has enough work to make 50 lines in the link, so they will employ pretty much 50 people. You will work each line in the link once every 50 weeks (and hours are usually averaged over the link - so you will work 35 hours average over 50 weeks, some weeks being much more or less). Generally with railway work you will get 5 days off once a month.
If Newcastle only had 49 members of staff then every week some work would be covered with rest day working from other staff. If they had more than 50 then the extra would not be in the link and be "supernumerary", just covering sickness and other staff incidents.