Couple of different ways crews work. First is the Extra Board, employees are on a list as they work, their name goes to the bottom of the list and they wait till their name gets back to the top of the list to be called. Some senior engineers and conductors work the EB because it can make a lot more money. Typically if the board is moving fast you might have 12 hours off before being called again. Today on NS the EB has days off for conductors only: work 6 have 2 off, work 6, off 2. Engineers just voted to allow this but it may take a long time to implement. If you work the road you will go out, spend the night (or could be day) in a hotel and return home the next day. Repeat in as little as 12 hours or if the board is moving slow, not go out for 2 days.
Other crews have assigned runs. A 3 sided pool means out one day, overnight (or overday at the hotel) and then return and typically have one day off. Then repeat.
Other crews work the yard, a yard job is usually 5 on, 2 off. Only senior guys will have Sat Sun off. A lot of yards are 24 hours so there is yard extra list that covers all 3 shifts. and then yard jobs you bid on based on seniority. Yard jobs also have assigned hours on each shift.
One other odd thing of RR life is the relief job: If you make first shift have Sat Sun off, second shift have M-Tue off and 3rd have Wed Thu off, well then you create a relief job that works Sat Sun first, Mon Tue 2nd and one of the third shifts. Or some variation of that could be 2 3rds, 1 second and 2 first shifts, etc.