There are about 115,000 people that are part of the freight rail unions.
Additionally, a number of these systems require a certain number of people to be able to go. It doesn't do much good to have one person show up to move stock around if the other people needed are out for some reason.
Picture the challenge of "ok, we want to go from 7 sick days a year to 14". That's 7 more days, which means that it needs to hire 4% more staff which amounts to hiring another 5000 people.
In today's labor market, this is hard.
Additionally, that's still running the 100% work utilization. Lets put some slack into that... now instead of 5000 more people you need, you need 15,000 more people.
Parts of this industry are licensed - you can't just go hire people off the street to fill those spots. They need between three months and year of on the job training and then some spots are under the licensure of the Federal Railroad Administration.