They're essentially coach during the day, but at night the bench-backs swing up to become beds (so, two side by side bench seats become two vertically stacked bunk beds). This does require some consensus on the train about when the "transformation" is supposed to happen (incompatible with hyper-individualism?). I also suspect that it is made more viable by India's extra-wide rail gauge (they can fit an extra, sideways bench on the other side of the aisle).
Eastern European trains are close to this, but they seem to have 2/3rds the capacity, because they don't have the sideways benches. Soviet rail gauges are again wider than the "standard" Western one, but perhaps not as wide as India's.
Let's see:
- "The 1,676 mm (5 ft 6 in) gauge is now commonly referred to as Indian gauge."
- "The standard Russian railway gauge of 1,524 mm"
- "The U.S. federal safety standards allow the standard gauge to vary from 4 ft 8 in (1,420 mm) to 4 ft 9 1⁄2 in (1,460 mm)"
So yeah, India has nearly an extra foot to work with.
Still though, I think Eastern European style cars ought to be possible in the US. It shouldn't be "sit upright in coach all night, or get your own private room", with nothing in-between.