Maybe one superseded the other? I wasn't aware there were two amps on the audio board. I wonder if there's a block diagram of those boards somewhere in the world. The pinouts are fairly easy to find with a Google search but I've never seen the block diagram. It's still a real shame Lionel took those TMCC boards off the support site.
Edit: the other 8 pin SOIC might be an LM358 op amp.