Green’s Dictionary of Slang

1. (US) ‘a small tram-car horsed by a single animal, and on which the only official is a driver, whose office it is to collect fares and generally perform the duties of conductor in addition to his own’ (Farmer, Americanisms Old and New, 1888).

1875		Chicago Trib. 8 Sept. 8/2: The bobtail cars ought to be taken off the streets right away, or conductors put on them [DA].
1882		A. Trumble Mysteries of N.Y. 62: Men [...] who are in the habit of riding next to the Slawson box in a bobtail car and accomodatingly putting in the fares of other passengers for them.
1884		Sporting Times 16 Aug. 5/5: Why is a bobtail car like the neck of an empty bottle? [...] Because, of course, it’s a non-conductor.
1889		Farmer Americanisms.
1890		Nat. Police Gaz. (NY) 29 Mar. 3/1: Capt. Blake always has a vacant knee or can pass up a nickel in a bobtail car for a lady without moving an eyelash.

2. (US tramp) a short local freight train.

1930

‘Dean Stiff’ Milk and Honey Route 2011: Peddler or bob tail — A short local freight train.