Narrating his encounters with specific drivers, Black Beauty uses a metaphor likening them to the operators of steam engines:
Then there is the steam-engine style of driving; these drivers were mostly people from towns, who never had a horse of their own, and generally travelled by rail. They always seemed to think that a horse was something like a steam-engine, only smaller. At any rate, they think that if only they pay for it, a horse is bound to go just as far, and just as fast, and with just as heavy a load as they please.
This comparison provides a window into the mindset of horse-drivers who treat animals like machines. Because they lack a deeper understanding of horses’ intelligence and ability to feel pain, they see them merely in terms of input and output.
To them, a horse's performance is expected to align directly with the amount of food or care it receives, much like a steam engine's output corresponds to the fuel it's fed. This perception, of course, totally erases the fact that horses are living beings with feelings. Steam-engines don’t care about being beaten or chastised, but horses—in this novel—certainly do.
Black Beauty is describing an unfortunately common phenomenon: a driver who ignores the nuances of a horse's well-being, emotions, and individuality. When Black Beauty tells the reader that the people who treat horses this way “generally travelled by rail” and that they mostly “never had a horse of their own,” he implies that they don’t know the difference between a horse and a machine. The central message of this passage is that treating horses like this is ignorant, and that a more nuanced and less mechanical perspective would be far better.