Druja-Dūkštas Railway post 1937

Well, unfortunately it is a damaged stamp... but I have my reason why I do keep this stamp preciously in my album. This stamp has a 1937 polish T.P.O. postmark of the line Druja (now Друя / Druya, Belarus) - Dukszty (now Dūkštas, Lithuania)! I have few books and documents talking about Polish railway post that operated in Vilnius region but none of them mention about this Druja - Dukszty line for some reason which I have absolutely no clue. According to the list of Polish railway post line, the line Druja - Dukszty existed and the number was 491 which supposed to be shown on the bottom of the postmark (it is missing on my copy). Philatelically speaking, that's about all the information I've got so far.
This narrow gauge line linked Dukszty / Dūkštas and Druja, the northernmost Polish railway station ever existed in Polish history, via Браслаў /Brasław (Breslauja in Lithuanian), the most populated town along this railway line. Originally, it has been built by the German during WWI which the Poles took over and converted to a 750mm narrow gauge line. There were some interruptions during WWII but the line was used until the late 1960's during the Soviet rule.

The line Druja - Dukszty on a 1929 Polish map

 Only 5 stations were actually in the present Lithuanian territory: Dukszty / Dūkštas, Berżeniki / Beržininkai, Rymszany / Rimšė, Biernaty / Bernotai and Czepukany / Čepukai (part of what used to be Czepukany extends on the Belarussian side as well). For more informations, look at the thread in miestai.net, the link is HERE