The pot is a 10k pot, the middle pin is the wiper. If the pot works backwards swap the outside leads.
GRJ,
I, too, was inspired to buy a couple of these boards this past weekend @50% off to add some sound to MPC-era diesels. As I read through this thread to get the wiring scheme, it seems by your comment that the three-pin connector is the sound pot (since a six-pin connector doesn't have a middle pin) - while Chuck's note seems to imply that is used for the speaker + on/off. Is that accurate?
Also, are there standard P/Ns for the harnesses? They aren't listed on the parts breakdown I found for the associated F-3 on Lionel's web site. Is there a "master list" of modern Lionel harnesses somewhere? I saw this thread of yours which gives info on the power connector - so down to just the three- and six-wire ones.
As a side note: it appears that the speaker specified is 0.2W at 8 ohms.