I had those too, loved them! My mother tried to pawn off some fake store-brand logs at first, they might have been called Link-And-Logs or something cheesy like that, and were made of pasteboard, so they quickly fell apart. A couple of years later, she stuck a crowbar in her purse and pried it open just enough to kick in for the real deal. ~
My momma was cheaper than your momma. She saved S&H Green Stamps and bought me a cheap train set that only went around in circles. I wanted a Lionel like my cousin had. (H&S green stamps were given away at various stores as a way to get you to come back. If you had enough, you could trade them in for gifts)