FfarquharDeveloper's TrackMaster Customs

That makes much more sense because other traction engines have it (except it's concealed on Trevor, George, Buster and Fergus). Turns out the part I outlined does move while the loco is running, but only ever so slightly and not like a coupling rod. Now that I've seen how the actual rod operates, it's more similar to the ones Rocky has to power his arm.