I was researching this song and found the following, From "The Hobo's Hornbook" (1930) by George Milburn (whoever he may have been):

. . . few tramps make any distinction between the rods and
the gunnels any more. The gunnels are heavy iron truss
rods that run lengthwise beneath a freight or passenger
car, and, on freights, are comparatively easy to ride*
The rods, or, more correctly, the rod, is found only on
four-wheel passenger trucks, and riding it is an achieve-
ment denied all But those who have thoroughly mas-
tered tramping. Between the cross-section and the axle
of the oblong four-wheel truck is a slender rod, little
more than a yard long, parallel to the partition and the
axle. On this the tramp once fitted the groove of his
ticket, a board as broad as a man's hand and about six
inches long. Crowded in this small space on the forward
truck (on the rear truck he must "punch the wind35)
the passenger stiff rides, a feat requiring skill and cour-
age. Few tramps nowadays know how to locate the rods,
much less dare ride them.