SP's TEBU, here mated with two mothers. Each GP40 could power one truck.
TEBU had it's own DB's. So 'I assume' the power from all four traction motors would go to the DB grids (top).
Slugs need a mother to control them, or provide DPU (or locotrol). If the slug used AC traction motors it would need excitation current to act like an alternator.
Link: [
www.locophotos.com]