Ordnungspolizei and Sicherheitspolizei would be used for train security under some circumstances. For instance, prisoner or KZ transports would be accompanied by Orpo or Sipo troops. It depended upon what was being transported or personnel staffing requirements.
Bahnschutzpolizei would have primary authority for Deutsche Reichsbahn asset protection while some militarized units could be in charge of passenger monitoring on trains and/or in train stations. Regular Orpo units might also be seen on trains or in train stations. No easy answer I am afraid.
Notice that the three men in the photograph are not security personnel but are armed railway employees. The two on the left appear to be wearing black Heer belts while the one on the right appears to be wearing a brown Czech belt. The early NS Zeit Reichsbahn wore Czech Cz27 pistols that were property marked to the Deutsche Reichsbahn on the trigger guard like regular Polizei pistols. Here are a couple of other weapons that are property marked to the railway.
