The early K-Line F-series B-units had two motors each, just like the A-units. An A-B-A set had six motors, really too much power - I know a guy that got rid of his K-Line WP A-B-A F-set because he had only 4 Madison cars to follow, and the engines moved at low power like there was nothing behind them. He could not apply enough power to blow the horn because the engines would then go too fast to stay on the track!
Once they changed to TMCC/RS in the A units, they moved the smoke feature to the B-unit, perhaps for space, and perhaps also because an A-A combo with four motors already had plenty of power.