MTH Railking 30-1503 Decapod had a Proto sounds 2 with a 3 volt board and the 2 AAs.
Selecting the support panel on the MTH site still shows the owners manual.
Page 18 shows the double AA s. This 30-1503 was the only Proto sounds 2 engine I found so far with this configuration . I'm sure there must have been earlier ones.🤔