PT 24 samples, lights 4 samples, everything else is default...
Scene is simple it's just what you see there(metro train car with characters and one empty car behind it). It is lit by a hdri and those interior lights...
I believe that the problems come from those lights. They're actual square lights inside onesided material glass meshes(onesided cause I wanted those lights look glossy through the lamp shading). I will try emitting light just from lamps and if it helps I will try textured emission...
Every material is reflective, windows are thin(twosided mat), there's displacement on characters(max poly subdiv 10 pixel projection 1px) and hair on caleb but in the end not much of geo there, I was rendering higher polycounts with corona this shouldn't be a problem I guess...