Sean, when we sold our traditional stuff off, I kept an ABBA set for sentimental reasons (it was my son’s favorite). Here is a not-so-great picture of it on the far corner of our shelf.
Both B units and the trailing A unit are dummy engines. They are identical to yours. You should be able to find the item number on Lionel’s site and then start looking for them. I think I found mine on eBay or dealer NOS ten years ago. If you can’t find the item numbers, let me know and I’ll look for the boxes.
Adding them to the base set - along with some MTH add on cars like the full length vista - really made for a great train.
Happy hunting.