How bidding and bumping works can vary by railroad and contract. You didn't say what line you ride and that probably does make a difference. My railroad service was over 30 years ago but where I worked, there were some practices I've never heard of elsewhere. For instance, for yard jobs, every so many months, everything expired and had to be rebid. OTOH, under many contracts, once you're on a job, its yours until you either successfully bid another vacant job or you get bumped and exercise your seniority to bump someone else (bumping means someone's job was eliminated and they now need to find another job. Generally, if you're holding a job, a senior person can't just bump you because they want the job. To bump, either their job must be eliminated or they have been bumped by someone else. In any event, a job elimination is what kicks off the bumping process).
Assuming the number of jobs are stable, generally everyone stay where they are until someone retires, quits, etc. Then that person's vacancy is bid. That opens up the successful bidder's job and that vacancy is bid. Lather, rinse, repeat (where I worked, everyone had to maintain a bid list of what they wanted so the chain of bids to replace a retiree, etc. all happened at one time. Only new positions were posted to give people a chance to update their bid list).
Currently, I ride the Milwaukee West. Under that contract (Metra), from what I've learned from talking to the trainman, in addition to the normal bidding and bumping, they can bid a vacation relief when the regular person is on vacation for a week. So a couple of weeks ago, my regular conductor disappeared for a week. I assumed he was on vacation until one day I took a different train and found him working it as a vacation relief job. Why bid a vacation relief job? To temporarily hold a more desirable job.
Passenger jobs tend to be stable. But freight service can vary considerably. In my day working for a railroad as an assistant trainmaster, I once had to make the call to emergency annul half a dozen full-crew jobs (all but one) in our yard due to a fire that temporarily shut down 99% of our yard business. That was a bumping mess but is unfortunately what can happen on a railroad.
Roselle, IL (along the MILW West line)
ex-N&W Sandusky, Ohio