Question: Where does Rusty go to be repaired, as he is the only narrow gauge diesel? Does the diesel works have narrow gauge tracks?
Answer: He probably gets repaired at Crovan's Gate Works.
Question: Most of the characters on this show are male. Why is that?
Answer: Female characters in literature, movies, television, etc. are commonly and historically less prevalent than male ones. It's an inherit gender bias that continues to exist within male-dominated industries. This same bias exists against characters of color and other orientations.
It's also a really old show that would've been fairly male run.
Answer: The diesel works don't have narrow gauge tracks.