A switchable powertrain includes a main gear set configured for gear ratio selection. The main gear set includes an input shaft, first gears coupled to a main shaft, and second gears coupled to a second shaft. The second gears are meshed with the first gears. The switchable powertrain further includes a first torque source, a second torque source, a clutch configured for selectively coupling the first torque source to the input shaft of the main gear set, and an adapter. The adapter includes a first selective coupling for forming a first adapter torque transfer pathway from the second torque source directly to the main shaft, and a second selective coupling for forming a second adapter torque transfer pathway from the second torque source, through the main gear set, and to the main shaft.