A subdivision map is probably not an “official government map”. It was probably drawn by the land development company’s surveyor.
Probably since the purpose of the map was to illustrate which lots were for sale, it was for marketing, the ownership of the railroad really isn’t that important. I’m suprised that they showed the railroad at all. Since the map probably isn’t intended to be used to establish title to the land they didn’t bother spending the money to do the search for the current successor to the railroad.