Is the vehicle depicted below some form of US military armored vehicle modified for use on railroads? Is it an MRAP so modified? Is it self-powered or simply able to be towed by a locomotive?
Almost certainly not.
Google and Tineye reverse image searches both trace back to this image on Flickr, which describes it as a "Padda" armored patrol vehicle used by the South African railways. Google image searching on variations of "padda armored vehicle" bring up similar-looking images such as this one. Perhaps there's mass confusion between a jerry-rigged rail MRAP and the "Padda" (there's precedent: the depicted vehicle has also been misidentified as a rail-adapted version of the Casspir landmine-protected vehicle), but more likely, it's exactly what the Flickr caption says it is.