Biden could reinstate them by EO, just like how Trump cut them. He hasn't though. There is a bipartisan consensus that the safety of the rail system is less important than rail company profits.
Any of us can buy railroad stocks and become - in your mind - wealthy. Perhaps the more important value being protected was, if the railroads stop working, many of us stop eating. Same reason we subsidized food production, fuel, etc. Our societies have grown too much to be able to go back to life without them, without huge numbers of deaths as we adjust.
He probably just hadn't noticed. It couldn't be that he allowed to rules changes to continue because some money flows towards him, his friends, or his party and his party's friends depended on maintaining those changes. Biden put a famously competent and experienced person in charge of Transportation and he didn't notice the change, so how could you expect Biden to?