What do travelers by express train say about the spectacle of Coketown’s factories at night?

From the text:

The lights in the great factories, which looked, when they were illuminated, like Fairy palaces—or the travellers by express-train said so—were all extinguished; and the bells had rung for knocking off for the night, and had ceased again; and the Hands, men and women, boy and girl, were clattering home.