We don't do much in the way of all-steel construction anymore: it wouldn't surprise me if the "rickety" steel approach would actually cost more than concrete in today's economy.

(Not that I think we'd look to build "rickety" new construction in any case.)

The elevated steel tracks have not "held up just fine" in Chicago. They've been neglected for decades, and they recently had to shut down a major line for almost two years just to perform necessary deferred maintenance--essentially, rebuild the line--to keep it go.

Which one?

The Red line between Cermak-Chinatown and 95th Street [1] was reconstructed over 5 months [2] in 2013.

[1] None of the reconstructed line was elevated steel structure.

[2] Not "almost two years"

Also, the Green Line between 1994 and 1996.

I was wondering if they were referring to the Green Line stuff, which I remember from that weird HODAR/LEJACK PR campaign CTA ran in the '90s.

I believe the Lake St line was closed for 2 years in the 90s.

Right, but that's not "recent".

Doesn't SF have significant earthquake hazards? That would explain the extremely heavy structural design.

I don't think that SF earthquakes are a major factor when planning construction in Washington D.C.

To be fair, if you just looked at the picture it would be easy to mistake DC's Metro for SF's BART system.

It seems quite possible that the concrete is cheaper.