It's often easier to start from zero.

It's also likely easier to move faster when the public is more forgiving of worker deaths. No idea on cumulative statistics, but the following headlines are illustrative:

SHANGHAI: Two men were killed and six others were injured here yesterday in two separate incidents at subway construction sites. [1]

5 dead and 18 injured in Shanghai Metro Line 12 construction site accident [2]

Not Shanghai, but...

On Nov 15, a subway tunnel under construction collapsed in Hangzhou, Zhejiang province, killing 17 people. [1]

[1] http://www.chinadaily.com.cn/china/2009-01/09/content_738040...

[2] http://www.exploremetro.com/blog/5-dead-and-18-injured-in-sh...

Can you explain why in this case? I don't really see how it can make that much of a difference. Shanghai was already a huge city, for example.

Building new tunnels in NYC takes a lot of careful work to avoid damaging the other underground infrastructure and above-ground buildings.

You're implying Shanghai is an empty city with nothing below ground. It's probably got just as much subterranean junk as New York does.

New subway lines have to connect into existing running subway lines. Digging the tunnels is usually the easy part, it's the points where they connect to things that is hard.

And that's why you think the 2nd Avenue Line will take 2-3 more decades?

After 20 years the Shanghai subway is now bigger than NYC yet they are scheduled to add a couple hundred additional miles.

No, I was not answering that question. I think Shanghai gets stuff done faster because it's in China. And because they actually have funding.

And it's not relevant to the NYC subway either so the comment that you made was irrelevant. I'm not sure why you wasted everyone's time.