Biden only picked up that ball after the East Palestine disaster vindicated all the reasons the union wanted to strike in the first place. It was a PR move because he had blood on his hands.
For comparison a single socialist city council member in Seattle pushed and won the for 12 days of sick leave for the entire city back in 2012. 4 days is bread crumbs. They would have gotten a much better deal had they just been left alone to strike and negotiate.