There was something I discovered about settling on a BG, I would have to look at the game though, it's not really significant, but it might make 1 turn difference. EDIT: I remember, I had two cities, one on a BG and one not, and they both had the same shield. That means that you lose the bonus of one shield when you settle on one. I have noticed that wherever you settle a city, it gives it one shield if it doesn't already have one, so you can maximize production by reserving you're BG's to get worked. Basically, the same grassland city will have the same 2 shields no matter what type it is on. So you lose the opportunity at a 3 shield square once you get rails.