Napthalene is a white solid derived from the by-products of coal gas production. It is the simplest polycyclic aromatic compound, consisting of two benzene rings fused together. It is a white crystalline substance that smells of mothballs; it was the main constituent of them. It melts at 78 degC, and is not at first sight an attractive fuel for any sort of engine. The page on solid-fuel IC engines shows that using napthalene in the form of a powder is not going to be easy.
The only napthalene-powered locomotive currently known was an experimental design built by Schneider-Creusot. In 1913 it began working within the Schneider-Creusot plant.
Left: The Schneider-Creusot napthalene-powered locomotive: 1913