" Although railway Flak units are part of the German Air Force and are administered through the usual GAF channels, it is probable that train protection detachments are operationally subordinate to the transport authorities; there is some evidence that guns provided for the protection of military trains may in certain circumstances be manned by army personnel."
If crewed by Luftwaffe personnel, waffenfarbe is bright red. Light flak (Heer) is “white with Fl on shoulder straps”. Source; Handbook on German Military Forces.
Luftwaffe Flak crews were also issued with plain black one piece coveralls to serve the guns. These I feel are a more likely uniform for a “static” unit, i.e. one that didn’t leave its location, in this case a train. Other Flak units including Waffen SS also received the coveralls on occasion.