I have seen shortlines scrambling to keep the pace... adding unit crude trains means that you have to add extra crews (jobs), because you are already running at maximum capacity with manifest tonnage.... drilling mud, propant, chemicals. I have also seen bottlenecks where inbound empties need to somehow pass the outbound loaded train... and they don't have a passing siding long enough, so they have to get creative with ducking trains out of the way on sidings that are long enough.... or running the empties up past the loadout facility, pull the loads and then set the empties in (which is good because you can get one crew to do the whole job).
The revenue these trains generate goes to pay off debt on previous CAPEX projects, and allows for infrastructure improvements. One day the oil boom will end (again) in the Permian Basin... fortunately one client of mine is in a position where they can transload potash... which will sustain the railroad for another 90 years.