I just took a dozen pots out of my parts box, they're from an assortment of Lionel and MTH locomotives. I picked the ones that had the Lionel connectors as they're from the Lionel equipment. All of these are from the modular Railsounds 5 or earlier era. Of the 7 pots, only one was an audio taper, and the other six were linear taper pots. Interestingly enough, all the MTH pots that were left were also linear taper pots. They may be specified as audio taper, but one wonders what is actually getting into the locomotive.
Mark, there's an easy way to determine what you have. Disconnect the pot, usually just pulling a connector. Turn the power all the way CCW and connect an ohmmeter to the center lead and to the outside lead that reads zero ohms or close to zero ohms.
Turn the pot about 10% of the travel, normally about 270-300 degrees of travel. Record the resistance. Repeat until you get to full scale.
If the numbers progress up fairly steady, it's a linear taper pot. If the first few are bunched together, it's an audio taper pot. I'm curious as to what you actually have.