- parts of the railway network were seized by Germany, most obviously in Poland from 1915 on
- military action damaged railway lines and rolling stock
- Russia could not produce and install replacement lines and rolling stock and imports were much reduced
- railway lines and trains were taken over by Armies (Imperial, Red, White) for military use
In areas which remained under Russian control, the TPO system never returned to the size it had grown to by 1914. Soviet TPO mail is actually quite scarce and you will not often see an item like the one below, posted in 1928 on the MURMANSK 36 LENINGRAD route:
In contrast, independent Latvia, Estonia and Poland re-created extensive and widely used TPO systems between the World Wars. Some of these offered Registered Mail services - see the illustration below from 1939 on the RIGA - ZEMGALE route:
During the Civil War period, TPO mail is scarce. In contrast, it is quite common to see Railway Station (Voksal) cancellations: perhaps people walked or rode to the local station thinking it would speed their letters if they posted them there. Mail bags could be loaded onto passing trains even if they had no TPO facilities.
For Transcaucasia in the 1917 - 23 period, you will occasionally see loose stamps with TPO cancels ( Baku - Batum, Dzhulfa - Tiflis) but covers are rare. For White Russian areas and Ukraine, the same is true. It is most unusual to see an item like the one below posted in 1918 on the un-numbered KHOLONEVSKAYA - SEMKI route (the Trident is Podillia 8b):