It is part of the third big wheel in the first picture that is missing the pictured part. I'm unable to fix it so I took it off the engine.
I will include the part with the sale. You will notice two pieces in the picture, one being the rim and the other was a little detail I found in the box and I'm not sure were it goes. Once I took this piece off I tested the unit using my Proto 2.0 system and it worked beautiful.I didn't notice any slipping of the wheels with the one traction tire missing, but I also have no grades on my test track. All the lights worked and it sounded awesome. This is a big unit and requires O-72 curves. All the features of this engine are highlight on the side of the box, so please review the picture detailing this information. In the box you will find the parts, as-noted above, replacement front coupler, new smoke fluid, 4 replacement traction tires and a hex wrench. Let me know if you have any questions. The item "MTH O Scale Western Maryland WM 4-6-6-4 M2 Challenger Steam Engine 20-3241-1 P2" is in sale since Friday, October 18, 2019.
This item is in the category "Toys & Hobbies\Model Railroads & Trains\O Scale\Locomotives". The seller is "mikesexpress" and is located in New Fairfield, Connecticut. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Sweden, South africa, Belgium, France, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Mexico, New Zealand, Switzerland, Norway, Ukraine, Croatia, Chile, Colombia, Costa rica, Panama, Trinidad and tobago, Guatemala, Honduras, Jamaica, Barbados, Bermuda, Bolivia, Egypt, French guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Cayman islands, Liechtenstein, Luxembourg, Monaco, Martinique, Nicaragua, Paraguay, Reunion, Uruguay.