Firing someone is a tricky business, and it happens more rarely than a lot of people think. Short of breaking a law while on the job, violations of safety policy, threats, or failing a d–g test, It is almost impossible for a company to fire someone, no matter how bad they are as an employee.
UP's termination process, at least when I was there, began with a reprimand, followed by an IDP for (usually) six months. after that the employee was reevaluated and a final decision was made to keep/dismiss them. EEO was also involved. Nobody working for UP full time can be fired without due process. Most companies (including UP) usually try to pressure the employee into quitting instead, but that has risks, and if the employee can show that this was the intent, they will have grounds to sue.