Hey, something maybe for one here I can shed some light on, for those who want a little more than the article gives. ;)
I am a supervisor on a commuter rail, and in this respect they operate in a similar fashion.
Like any other business, profits are key. Someone figures out staffing. Pretty much cut and dry. For a line from terminal A to B, there is one train per hour in each direction, 48 trains in a day. We need for example a supervisor at each end, and one engineer and conductor per train. So, say that gives us 48 train crews, and 6 supervisors per day. So, we hire 6 supervisors, and 51 train crews - in case someone calls out sick.
What happens if someone calls out sick? A supervisor? We keep costs down there by requiring the on duty supervisor to stay an additional shift. A crew member? We have that extra crew. If we have too many people call out sick, we force on duty crews to stay on overtime. We don't want to be doing that, now do we? That affects the bottom line.
If no crew is available at all, then the train doesn't run. This is also bad, as it upsets the customer.
Running with a "skeleton crew" ensures we are not spending more on labor than we have to. This is because in some markets, paying fewer workers overtime still comes in at less than having more people to whom benefits are paid.
So, the employees are intimidated into coming to work every day.
What the freight railroads are doing here is forcing the employees to use their vacation pay when they are sick to discourage them from calling out sick at all, in effect penalizing them for calling out sick. That vacation you had planned with the family? Either lose a day (which may require you to cancel altogether), or come to work sick. If they have paid sick leave, they'd use it when sick rather than showing up to work.
My agency uses a different tactic, just as effective in the employer's view, but no less exploitative of the employee. When an employee calls out sick, a Special Inspector may be sent to their home to check if they are really sick and at home. If the employee doesn't answer the door (They're sleeping and don't hear the door? Doesn't matter), they're charged with falsely calling out sick, and face suspension or termination.
Edit: This industry can be very time sensitive. It may not seem to you a big deal if say a train is late, but cargo is critical or a passenger gets to work late, that is not a good thing. The shipper may take some action if it happens enough. That passenger may start driving.