They're probably just not set up for it. It takes time to do the necessary surveys and testing and report preparation. It takes less time, but still a lot of it, to write contracts, send them out for bid, and choose a consultant to do the work. The government at all levels is already understaffed thanks to years of hiring freezes and budget cuts, and I can guarantee the Trump years did a number on people's willingness to join the EPA.

Plus, what do they do when Norfolk Southern stalls on paying the bill? Add higher interest rates? I mean, sure, but levying huge fines while they get dragged through the court of public opinion would be worse than just a bill they refuse to pay while everyone else forgets about the incident.