I find that a lot of folks tend to lean on every automatic function they can find on modern camera. There is some level of risk in doing that, however. In the case of the AutoFocus Face Recognition capability, that may be fine at weddings and parties, but I would turn it off when walking around a railyard. I typically use single-point AF, in which I designate the spot in the frame where the focus should be. The camera still does the focusing, but I am in control as to where. On the Nikons, the AF-A or AF-C modes work best for railroad shooting, because they will adjust if the subject moves.
I can definitely see some potential problems for a shooter who uses both the face recognition feature and the shutter priority feature for exposure. If the camera focuses on the wrong subject and due to less than perfect light, it selects an aperture such as f/4.5 or something even wider, the shot's going to look soft, or even blurry.
Can you tell I am not a fan of excessive automation?
/Kevin