Planet money also did a podcast on this and went into a bit of how the ACH system works (not from a development point of view). It's a very strange system and honestly the episode raised more questions than it answered but it seems there is a lot of secrecy behind how it all works. Also why it all works... The fact that it can take 3-5 business days to transfer money electronically between different banks in the same country is difficult to grasp, but they make an effort to explain why...

http://www.npr.org/blogs/money/2013/10/04/229224964/episode-...