In Begins, Gotham has a futuristic/fictional and run down look: think of Oldtown, the trains built by the Wayne Corporation etc. 

In TDKR, Gotham looks like plain old real world NYC - we don't see the trains anymore. Oldtown is just a standard poor neighborhood. Lots of the locations used in the movie are easily identified as real world NYC locations: the stock exchange or East River bridges for example. 

My question is: Why did Nolan change Gotham city from a shady fictional more cartoon-like town to plain old NYC? 

**Edit:** The examples from Begins and TDKR were just for illustration; I didn't intend to limit the question to those two movies.

**Edit 2:** I specifically want to know why Gotham was changed from a cartoon-like city in Begins to an almost real-world NYC in TDKR (via not so cartoon-like but not yet real-world like Chicago in TDK). Why it's NYC or Chicago is not so important for me. But why, e.g., were the trains which featured prominently in Begins gone in TDK? There was only a very short in-universe timespan between the events of the two movies, yet Gotham changed drastically.