Henry Grueber's UK patent 12193 of 1894 gives details of how these pieces were made. The patent's title is "Improvements in the Manufacture of Medals, Medallions, and the like, and in the Materials employed therefor." The filling between the two bits of metal that make up the obverse and reverse varies, and the in the patent he says "I fill in with, or cause to be occupied by, any suitable solid or malleable substance, which may be cardboard, millboard, lead, pitch, or any adaptive equivalent therefor."
My specimen of this piece weighs in at at hefty 18.8g so is probably pitch filled.
It should be pointed out that the wheel only had thirty cars and not the thirty five depicted on the medallion. There were six loading stations, i.e. one fifth of the wheel. (The wheel at Earls Court had forty cars and eight loading stations.
In addition to the two pieces shown there was a third piece using the four generations die. This comes in at a very lightweight 4.7g so probably cardboard filled.