Yes, it's a railroad cancel. It is probably from a parcel. It was carried on Train 434. The two town names on the cancel will be the route that the train ran.
you guys scare me...but to change the subject...when you get to look at items that have been stored away 50/60 years...stamp values have gone up...i am not counting inflation.
Just could not resist looking up the towns and railway lines on Wikipedia. Rothfließ was indeed the junction of the lines to Allenstein, Heilsberg, Korschen, and Lyck. It was the latter one that the train must have travelled, but not all the way. At Sensburg the line to Rudczanny branched off. The latter was closed already in 1945, and hence does not appear on the post-war map.
Usually, German railway postmarks look the same for both directions of travel (i.e. the place names are not swapped around), and the direction can only be identified from the train number. Even numbers were used for trains travelling from east to west and from south to north, so this cancel was apparently used on a Rudcanny - Rothfließ train.
