(Oxford English Dictionary)
main stem, n.
Chiefly N. Amer. colloq.
a. The main line of a railway.
b. The main course of a river.
c. The principal street of a town (now the usual sense); spec. Broadway, New York.
1832 Amer. Railroad Jrnl. 1 804/2 The western fork..connects it with the main stem.
1854 Debow's Rev. July 19 The main stem, besides throwing off large boyous [sc. bayous], divides with the Northeast and Southeast Passes.
1869 Bradshaw's Railway Man. XXI. 426 Assets. Main Stem..Lebanon Branch extension..Richmond Branch.
1869 Debow's Rev. May 360 It [sc. the Illinois Central Railroad] has its main stem, and its branches of twelve hundred miles.
1900 'J. FLYNT' & 'F. WALTON' Powers that Prey x. 250 Investigations that have been begun in 'the main stem'.
1907 J. LONDON Road vii. 160 The kids began 'battering' the 'main-stem'.
1927 W. WINCHELL Primer Broadway Slang in Vanity Fair (N.Y.) 27 Nov. 134/4 Broadway is known as 'The Main Stem'.
6 March 1922, Daily Northwestern (Oshkosh, WI), pg. 10:
So there is night life in New York yet. The old main stem of Broadway may seem to be making forced motions on the flaccid inspiration of strawberry soda when first observed by the visitor from west of the Jersey shore.
29 September 1926, Variety, pg. ?:
At first thought, this premise packs a punch but, as Joe Lee, the erudite Oriental philosopher, interprets it, "Broadway" will make the hicks expect the worst and their reaction upon being disappointed and finding themselves unmolested and treated civilly, will prove a boost for the Main Stem.
27 November 1927, Vanity Fair, "A Primer of Broadway Slang" by Walter Winchell, pg. 134, col. 4:
Broadway is known as 'The Main Stem'. Abel Green, a theatrical reporter, calls it "Mazda Lane" and others refer to Broadway as "The Incandescent District"; "Tungsten Territory", "The Big Artery", and "Coffee Pot Canyon".
By Samuel Marx
New York, NY: Donald Flamm
You may have never heard the name of some of them. But they are part, particle and parcel of New York's Main Stem, Hardened Artery, Great White Way, Incandescent Lane, Mazda Boulevard, Chow Mein Stem, Double Cross-Roads of the World, Two-Times Square, or, as somebody once called it, Broadway.
Old Fulton NY Post Cards
25 May 1936, Syracuse (NY) Journal, "On Broadway" by Walter Winchell, pg. 9, col. 2:
Broadway is known as "The Main Stem." Abel Green, a theatrical reporter, calls it "Mazda Lane" and others refer to Broadway as "The Incandescent District," "Tungsten Territory," "The Big Artery," "Coffee Pot Canyon."