▁United ▁States ▁Mail ▁was ▁performed ▁by ▁the ▁rail ro ads . ▁ ▁Spe cially ▁equ ipped ▁Railway ▁Post ▁Office ▁( R PO ) ▁cars ▁were ▁a ▁part ▁of ▁most ▁passenger ▁trains , ▁the ▁cars ▁staff ed ▁by ▁highly ▁trained ▁railway ▁postal ▁cler ks ▁who ▁sorted ▁mail ▁as ▁the ▁train ▁sp ed ▁along ▁its ▁route . ▁ ▁The ▁growth ▁of ▁this ▁mail ▁distribution ▁network ▁par alle led ▁the ▁expansion ▁of ▁the ▁rail ro ads , ▁allowing ▁mail ▁to ▁be ▁ex changed ▁between ▁routes ▁at ▁j unction ▁points ▁where ▁two ▁rail ro ads ▁crossed ▁or ▁shared ▁passenger ▁terminal s . ▁▁ ▁Short ly ▁before ▁W . W . I , ▁the ▁Post ▁Office ▁Department ▁began ▁assigning ▁railway ▁mail ▁cler ks ▁to ▁new ▁Transfer ▁Office ▁positions ▁which ▁were ▁established ▁at ▁many ▁of ▁the ▁larger ▁rail road ▁j unction ▁points ▁as ▁well ▁as ▁passenger ▁terminal s ▁in ▁larger ▁cities . ▁ ▁These ▁transfer ▁cler ks ▁super vised ▁mail ▁ex changes ▁between ▁trains , ▁served ▁as ▁the ▁local ▁l ia ison ▁between ▁the ▁Railway ▁Mail ▁Service ▁and ▁the ▁host ▁rail road , ▁and ▁maintained ▁detailed ▁statistical ▁records ▁which ▁were ▁used ▁to ▁aud it ▁rail road ▁charges ▁for ▁mail ▁transport ation ▁and ▁car ▁usage . ▁ ▁This ▁latter ▁task ▁involved ▁determ ining ▁the ▁amount ▁of ▁mail ▁in ▁b agg age ▁cars ▁or ▁storage ▁mail ▁cars , ▁a ▁unique ▁process ▁of ▁estim ating ▁the ▁car ▁area ▁occupied ▁by ▁mail ▁s acks ▁rather ▁than ▁counting ▁individual ▁s acks . ▁ ▁Other ▁duties ▁included ▁sorting ▁letters ▁which ▁had ▁been ▁ma iled ▁at