When it comes to acknowledging mail, RailCorp are on the ball. Within 3 days of posting my letter, a response came back - the fastest turnaround of any company that I have dealt with to date.
They haven't given me an answer, but they have committed to providing one within 15 working days. This is as it should be - I don't expect an organisation of that size to fix the problem overnight, but they acknowledged my letter as quickly as you can (their response probably went out the same day they received my letter) and have stated the time by which they will have an answer.
Now all they need to do is actually clean up the graffiti on their infrastructure, and I'll be happy.