What type of publication was a "railway circulation" book?  Clemens refers to
this in a letter of May 7, 1884 to Charles Webster writing that he is thinking
of publishing a "small book for railway circulation"?

Only  a few years earlier he had written of suing Ogilvie publishers over
a  railroad series  book entitled _Bad Boy's Diary_ with illustations by Merle
Johnson that Ogilvie had evidently hinted was the work of Clemens and which
Clemens disavowed.

What exactly were these "railroad series" or "railway circulation" books
and how were they marketed?

Barb