I don't completely agree with his premise that it was tradition to decorate with antlers. Rather, since each engine was assigned to particualar engineer, the tradition was to decorate it as the engineer saw fit. Photos are out there showing lace curtains & bud vases in one particular locomotive. Aside from the antlers in this photo, it also has a striped awning over the engineer. Fun photo.
Of course, that ended for the most part when the engines were pooled.