Crossrail is not directly comparable, for starters, only 21km of crossrail is underground; Most of it's 118km length is above ground so the per-km cost of the below ground sections get averaged out over the above ground sections.

This section of the 2nd Avenue subway (it's just 2.7km long) is deep under Manhattan itself. To get a comparison on cost you would have to only compare only the section of crossrail running under central london itself.

Only a small portion of that 118 km will be in newly built tunnel and there will be relatively few newly built underground stations on Crossrail, which is why it's much cheaper (underground stations are very expensive to build). Also, the use case for Crossrail is very different from the use case of the Second Avenue Subway.

London is built on clay and is easy to tunnel. NY is built on rock.

Hong Kong is mostly granite, which is significantly more difficult to tunnel than the shale-like schist bedrock of manhattan...and MTR construction reflects this fact, being more expensive per km than London, Paris, or Madrid. Even after this fact, MTR construction is still 50% cheaper than the average subway construction costs in the US.

In addition to the other comment-- As far as I understand most London trains are not 24/7. See the other HN post about this, but building 24/7 tracks is significantly more expensive because you need to build multiple independent tracks so that work and maintenance can be done on one while trains run on the others. This in turn requires more switches and complexity.

For NYC the trade off is worth it. The city absolutely requires 24/7 public transport.

Several lines are now 24/7 (fewer trains at night, but the service continues all night), with more to come.

I'm not sure if the crossrail will be in at night, but I hope it will.

They're not yet 24/7. They run through the night only on Fridays and Saturdays which still allows TfL to do maintenance on weekday nights.

Ahh yes, you're quire right.

That's cheap! Melbourne is planning the Metro rail project - a 9km tunnel under the CDB with 5 new stations. Current cost is approx $9-11bn AUD.

https://en.wikipedia.org/wiki/Melbourne_Metro_Rail_Project

Wow! Here in SF they're building cross-town BRT for $5m/mi. Blows my mind that London's subway costs that much to extend, dozens of times more expensive. And we have earthquakes to worry about, London doesn't.