Sometimes the outer edges of the frame (i.e. the walkways) appear to sag or be bent, but generally the inner part of the frame is fine. Sometimes the camera angle, or lens distortion, makes it look worse than it really is.
FWIW, I've seen this on many locos and cars, some of them much more severe than the ones you linked to. Photos can really exaggerate the visual effect, especially if taken from far away with a zoom lens.