Speaking from a traincrew pov this is what happens but will be dependent on local and toc specific agreements .
But at a small railway signing on point, or a remote signalbox, what happens when someone rostered calls in sick. Somewhere small. (I am thinking of somewhere like Skipton where 20 drivers and 20 conductors will be signing on early. or remote locations like Garsdale or Kirby Stephen signalboxes)
Do they ring round trying to get people to work an extra shift? or have spare staff? or just cancel trains.
So say Im due to book on tomorrow at 05:00 and ring in sick now Sunday evening at 21:00 the shift manager will try and cover my work tomorrow .At my TOC There are a ratio of standby train-crew who cannot be marked to any work on the roster posted 4 days before the day in question . So if there is a member of standby train-crew who can catch all of my job they will be given it when they book on . If not they might be given part of it , and another spare on the day or standby guard given it . Sometimes on standby you might be offered financial inducement to stay and cover a full uncovered turn if it doesn't take you over 12 hrs or within 12 hrs of your next booked turn . Obviously this is down to personal preference and the desperation on the company side for cover
There might also be a number of guards available spare on the day for a variety of reasons such as being spare in the roster and have not been moved to a job , just resumed from sickness so someone else has been brought in rest day to do their job or they might not sign their job so have been marked spare .
If all standby and spare cover is utilised at the depot then if other depots guards sign the routes and traction involved then cross cover can be asked for from other depots .
As a last resort if no cover can be obtained then the trains uncovered will be cancelled .
And what effect does it have when someone is going to have to be off for a month? They could throw the rosters into a complete mess.
(And I know rostering is difficult because of different traction and route knowledge)
Once it goes beyond the first day of sickness rosters will then endeavour to cover my work . So as well as standby turns we also have spare turns fixed into our base roster . With spare turns we can be moved around and marked to longer jobs upto 4 days before the day in question when the daily sheet is posted so fairly often when you are shown spare in the link you will actually find yourself marked to a full job to cover AL/Sickness/people restricted from duty etc . Rosters can then also bring people in on rest day to cover the turns .
If there is a shortage of available cover then my job would be uncovered and unless they can be covered on the day as above trains will be cancelled
lastly, what happens when a bug goes through a signing on point? Losing some of your staff due to a tummy bug must also play havoc with rostering.
Ive known of cases of it happening at big locations and even with a lot more spare and standby cover it can cause localised issues running trains . Particularly with routes or traction that only one depot or a small number at that depot sign .