Curiously enough, these two work cars are both recorded as being number 501.  One might conjecture that the first one, the cab-on-flat design, is the older; that photo is undated.  The second photo, of a box cab design, is dated May 9, 1942, which puts it in the era of the last-surviving electric freight service in Biloxi, long after rail passenger service had ended.  The box cab motor appears to be capable of taking on less-than-carload freight, and also of acting as a locomotive to move short strings of standard railroad freight cars.  A close inspection of the trucks and other under-floor devices suggests that these are the same car under the floor, i.e., that the newer one (probably the box cab) was built out of the older one, likely in the company shops.