Green’s Dictionary of Slang

(US tramp) the Missouri, Kansas and Texas Railroad.

1890		‘The Dream’ in J.F. Dobie Rainbow in Morning (1965) 160: I made a good run way up the track, / But they cotcht me on the Katy, an’ he brings me back.
1915		H.F. Day Landloper 33: Have you [...] beaten the face off the Katy Shack when he tried to pitch you off a gondola-car?
1925		G.H. Mullin Adventures of a Scholar Tramp 258: I caught a rattler on the M.K.& T. (the Katy).
1927		C. Sandburg ‘Po’ Boy’ in Amer. Songbag 32: Away out on the prairie / I stopped that Katy train.
1930		‘Dean Stiff’ Milk and Honey Route 23: Take the M.K.T., or the ‘Katy,’ the Bible-belt railroad of Missouri, Kansas and Texas.
1939		Howsley Argot: Dict. of Und. Sl. 28: katy [...] a train.
1970		(con. 1930s) E. Thompson Garden of Sand (1981) 144: An old switchman off a Katy caboose [...] popped his knob out the window.