As a LIRR employee, your seniority (date of hire and or date of qualification) places you on a list of fellow employees within your department: enginemen, conductors, block operators, ticket clerks, etc.
You bid the individual jobs and/or runs you wish within your area of employment. The newer people (low on the senority list) do not get the good jobs. The older people (higher on the senority list) get the better jobs.
The oldest person usually has their choice of jobs and usually gets the cream of the crop!!
Should you happen to get lucky and bid on, and win, a decent job, it's yours until a person of higher seniority comes along and says, "hey, I like that run . . . it will get me off closer to dinner time" or "it's more convenient to where I live. . .", etc., etc., they exercise their senority and bump you out of the job (run or position, not your job on the entire railroad).
Now, you don't have a job, so, you bump the guy who has less seniority than you and get his job. He, in turn, bumps the guy below him, etc., etc.
It's a pecking order that doesn't end until you're #1 or #2 on the roster!
Jobs used to bid weekly. I don't know about nowadays.
Dave Keller