I had read that the Biden administration kept pressuring the railways behind the scenes after the strike was averted till the unions got what they had wanted in the first place anyway.
I don't know where I first read it but this link seems to confirm it.
“We’re very happy about this. We’ve been trying to get this for decades,” said Artie Maratea, president of the Transportation Communications Union. “It was public pressure and political pressure that got them to come to the table.”