My posts on comparative rail construction costs are scattered across many posts, written over five years. They’re all tagged as construction costs, but so are posts that discuss a single project rather than offering comparative data. To simplify references, here are links to all past posts that aggregate construction costs, in chronological order:

US Rail Construction Costs: the original post, comparing subway construction costs in New York and a few other first-world global cities (2011/5/16).
Quick Note: Barcelona Rail Tunnel: one project from Barcelona (2011/7/29).
Construction Costs: Third World Edition: the original post about third-world subway and el construction costs (2011/8/22).
Relative Costs of Transit Construction: a discussion of subway and light rail costs, mainly in Canada but also in France (2012/9/25).
Construction Costs and Perceptions: a discussion of costs and perceptions, with data from the US, Switzerland, Singapore, and South Korea (2012/10/28).
Comparative Subway Construction Costs, Revised: another large database of subways, in both rich and poor countries (2013/6/3).
Monorail Construction Costs: exactly what it says on the tin can (2013/8/24).
Hyperloop Costs: exactly what it says on the tin can, but bear in minds costs seem to have run over since (2015/12/2).
Why Costs Matter: some overlap with older posts, but new data of subways in the US and Sweden (2016/1/31).

Many of the links, especially in the older posts, may be dead. Email me if you find a dead link and I’ll try to look for a live version.