The site content mechanism is explained at length here, single post and most of the rest of that topic:
As for Armorama vs Railroad:
Yes, it is used at the railroad.
To load armor on railroad waggons.
My very personal opinion is that the usage overrules the fact that it is used at the rail head.
An SSyms could also be used to transport other heavy stuff so in that case the railroad aspect dominates.
A railroad bridge used as diorama accessory to display tanks driving under it? Diorama, Armorama or Railroad? What if the bridge has lost the tracks and is used as a local path for sheep, cows, peasants and tourists? Railroad or diorama? The bridge was built for the railroad and then abandoned.
Is your scene of the Stug loading/unloading a diorama, armor or railroad?