I've seen a lot of concrete ties in some urban-rail systems, like BART and various light-rail systems. Interestingly, their track switches mostly have wooden ties, possibly because it's easier to construct switches with them -- easier to hammer in a spike or make a bolt hole at some suitable location than with concrete. But I've found concrete-tie switches in a few places, like a bit at the Hoboken Terminal.
Interestingly, the older lines of some recent light-rail systems have wooden ties, while just about all their newer lines use concrete ties.
I checked on Portland, Sacramento, San Jose, Los Angeles, San Diego, Salt Lake City, Denver, Minneapolis, St. Louis, Dallas, Hudson-Bergen, and Baltimore.
It must be said that it's easy to tell with Google Maps which ties are wood and which ones are concrete, at least in urban areas.
As to wood vs. concrete, could the issue be one of initial vs. continuing cost? Concrete ties are more expensive than wooden ones, but they seem to need less maintenance.