Railway circulations books were cheap paperback editions marketed through news
vendors.  They were precursors to paperback books and sold much as paperbacks
are in airports now, though in smaller stands and with less elegant covers.
They weren't sold only in railroad stations, but such vendors did a better
busness in books than most news vendors because people wanted something to read
on the train. Better Twain than the penny-dreadfuls also available.  SLC did
not like being the alternative to trash.
                                           Andy Hoffman