Mark, Best estimate for timing on this Loco is 1917-1918. The 2 piece lightened frame, Die cast 8 spoke wheels, reversing switch mounting plate design, hook coupler with nibs, lighting post on the top... These are all clues to the dating. Im expecting the windows in the end doors to have brass plates that say "MADE BY THE LIONEL MFG CO." on them and the motor to have 9 field laminations, but you did not show either in the photos posted.
The hollow shell was indeed a resonator and amplifier of the sound!