After the Bangor & Aroostook inherited most of the Delaware & Hudson’s old DL17s, some were set aside for conversion into control slugs. From the electrical compartment forward they look like their donor locomotives, but the machine rooms & radiator compartments have had their doors & vents plated over. Regardless of the HP rating of their donor locomotives, the SLG4s are all rated for 2000 (parasitic) HP.

14 have been built; three for the Ontario Southwestern, four for the D&H, and seven for the Diesel Locomotive Pool.

On the OSW & D&H, these slugs are known as pilotfish, because they are typically used as the control cabs for class K motors.