What is rarer in USA, a roundabout or a railroad?
Freight rail is reasonably healthy in the US, but it's mostly between larger cities. Passenger rail works in the northeast (Washington DC to Boston, roughly), but is much less viable outside of that with the exception of a few isolated lines (Southern California, maybe around Chicago). So where I am, for instance, there's a smaller (Class II) regional line that handles freight, but the major railroads have lines north and south of here that handle much more freight, and what little Amtrak traffic there is runs on the southern line (no passenger service here - you have to drive about an hour south for the nearest Amtrak station).
Roundabouts are gradually showing up here. (Having grown up without them, I hate them - put in stop signs or a signal.)
I don't have either of these on any normal daily drive, but it would be easier for me to find a rail line (but not a train) than a roundabout. There are some places where roundabouts have been built (either new development or revision to existing roads), but none are particularly close. I suspect this is generally the case over much of the US - while a lot of rail has been removed since the late 70's/early 80's, it's still more common than roundabouts, but that may flip before much longer.