Maybe one of our AF experts can provide the answer.
While I don't consider myself an AF expert I can look up information as well as the next guy. Here is a quote from Greenberg's Guide to American Flyer S Gauge , Volume II edited by Joe Deger. Page 101:
Rubber Roadbed Development
The idea for rubber roadbed started with a letter to the Gilbert factory from a man who owned a restaurant in Denver, Colorado. The letter stated that there was an American Flyer train running on track built on a shelf completely around the restaurant (out of the reach of children). Unfortunately the train created an objectionable level of noise and the owner wanted to know what Gilbert had available to quiet the rumble. According to Dr. Edward Bernard, Maury Romer in an effort to help the Colorado man out, started to cut some strips of sponge rubber and bevel the edges. While he was in the process of working on this idea, a salesman and part owner of the Bond Rubber Company of Derby Connecticut, visited him in engineering. The Bond Rubber Company already did some rubber molding for the Gilbert Company (primarily) the rubber tires for Pull-mor Power.)
After Romer explained what he was up to, the salesman offered to mold what he was working on. $500.00 was authroized to construct a single mold, instructions were given that the edge should have a stone effect, and that the track should nest in the roadbed without shifting.
A single cavity brass mold was mad and, after sever inspections and repeated chiseling of additional freatures into the mold it was accepted. Ties, in addition to the four track ties, were just high enough so that they could be stampled black. Although intended for this it was never done.
Two 24 cavity straight track molds and two 24 cavity curved track molds were made; the cost was quite high. Cartons were printed with A.C. Gilbert labels. Bond Rubber Company produced the roadbed, packed 48 to a carton, and sent them to New Haven. All Gilbert had to do was slap a shipping label on it and send it out. Gilbert got the roadbed for seven cents each, it sold for 35 cents in the catalogue. Profits ranged from fifteen to seventeen cents depending on how it was sold: to a jobber, a wholesaler, or a large department store.
The rubber roadbed was on of the highest profit items the A.C. Gilbert Company ever had. For years it sold by the "millions" since there had been many sets sold in prior years, Gilbert could not keep up with the demand. It was an item on which Gilbert made a lot of money and served a great purpose on train layouts as well. (This information on the rubber roadbed was originally published in The Collector)"
The numbers for the rubber roadbed were 726 for the straight sections and 727 for the curved sections. It was cataloged from 1950-1956. It came in Gray rubber with six ties, Black rubber with six ties, and Black rubber with 23 ties in 1956."
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