He stopped their strike to avoid crippling the US economy and kept supporting the unions in their contract negotiations until the unions got the sick leave they were asking for.
From the IBEW:
“We’re thankful that the Biden administration played the long game on sick days and stuck with us for months after Congress imposed our updated national agreement,” Russo said. “Without making a big show of it, Joe Biden and members of his administration in the Transportation and Labor departments have been working continuously to get guaranteed paid sick days for all railroad workers.”