As has been said, Extra Board OBS are "On Call" , while OBS with Seniority bid their jobs on a Schedule or as they become available.
I've run into many OBS on LD Trains that I've seen on several different Routes through the years, many serving in different Roles ( ie Food Service Attendant,Coach Attendant,SCA and LSA in the Cafe and Diner).
I've seen one Senior Chicago based SCA on the Empire Builder,Zephyr and Texas Eagle several times each( and he's one of the Laziest SCAs going!The Original "Invisible Man")
And an Infamous Food Service Person was moved from the Texas Eagle to the Zephyr because of the Numerous Complaints from Passengers about her Attitude and Poor Service.
One of the Best SCAs I've ever had would always work lots of OT on the SWC Route, she called it her "Christmas Money!"
Also,IINM, New Hires are cross trained so they can serve in different positions ( Coach attendant,Waiter,SCA)as needed.
When the Schedule for the Sunset Ltd. was changed to eliminate the Layover in NOL, several of the "Old Timers" bid the Starlight and told me they they much preferred the LAX-SEA route over the LAX-NOL Route.