Additional information provided by the "grandoldtoys" website. The Owner (collector) notified me that this was the first time the track was assembled just for the pictures. Now here's the Mystery part.
Attached to the interior of the tunnel roof is a grooved, plated steel ramp. One end of the ramp is hinged and fixed to the ceiling.
This allows the opposite end to raise and lower and works as a release mechanism. Hanging down from the center of the ramp is a lever. One car is wound up and placed underneath the ramp between the lever and the end of the ramp while its running.
The idea is that as the second car enters, it'll strike the lever, raising the ramp, to release the first racer. That's the Mystery; one car enters while another leaves. The real mystery is how to keep the first car running while the second is being placed on the track.
With start/stop switches on the rear trunk of both vehicles the tunnel car has to be inserted while it's running. You then have to have enough time to place the second racer behind it. If you desire additional pictures, let me know! The item "Marx USA Mystery Tunnel Windup car set Original Box Exceptional Condition" is in sale since Wednesday, July 3, 2019. This item is in the category "Toys & Hobbies\Vintage & Antique Toys\Wind-up Toys".The seller is "a1095" and is located in Menomonee Falls, Wisconsin. 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, Japan, China, Sweden, South Korea, Indonesia, Taiwan, South africa, Thailand, Belgium, France, Hong Kong, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Israel, Mexico, New Zealand, Philippines, Singapore, Switzerland, Norway, Saudi arabia, United arab emirates, Qatar, Kuwait, Bahrain, Croatia, Malaysia, Brazil, Chile, Colombia, Costa rica, Panama, Trinidad and tobago, Guatemala, Honduras, Jamaica, Antigua and barbuda, Aruba, Belize, Dominica, Grenada, Saint kitts and nevis, Saint lucia, Montserrat, Turks and caicos islands, Barbados, Bangladesh, Bermuda, Brunei darussalam, Bolivia, Ecuador, Egypt, French guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Jordan, Cambodia, Cayman islands, Liechtenstein, Sri lanka, Luxembourg, Monaco, Macao, Martinique, Maldives, Nicaragua, Oman, Peru, Pakistan, Paraguay, Reunion, Viet nam, Uruguay.