It is not surprising that a branch line upgrade should be so expensive. Even the heavily used main lines in the U.S. are still mostly wood ties, for example, so if you have to upgrade the entire ROW to concrete ties, and then put in a whole bunch of high-tech crossing gates, and new stations, you are essentially building from scratch. The only cost you are avoiding is the land acquisition and basic grading.
I'm guessing there was a standard to meet that was more appropriate to the task at hand, so the approvals delays were much less. I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.
