No. I'm afraid it's not possible to use both features at same time with V-Ray GPU.
UVW can be embedded in the geometry disabling the instance engine. But then, V-Ray doesn't recognize the object as a RailClone object (but a mesh), and doesn't apply RailClone Color.
An alternative is to use VRayUVWRandomizer together RailClone Color. In this way: RC Color > VRayBitmap > UVWRandomizer (in the Mapping Source slot). Also Instance Engine must be enabled in RC.
In this case, RailClone UVW options have not effect (including both Segment > Mapping and UVW XForm node). Mapping is handled exclusively by the VRayUWVRandomizer.