I'm having trouble understanding the meaning of these words at the end of the second chapter of The Adventures of Tom Sawyer. The narrator reflects on what Tom Sawyer has learned in manipulating all the other kids to whitewash the fence. He cites how wealthy men in England pay to drive coaches for the fun of it, and he says they drive them "twenty or thirty miles on a daily line, in the summer".
I don't understand the usage of "line"—I'm thinking that maybe it is referring to line in the sense of some section in a route. However, this may as well be an idiom, but I cannot find any other usage of on a daily line in any other body of work.
Following, the paragraph in question:
Tom said to himself that it was not such a hollow world, after all. He had discovered a great law of human action, without knowing it—namely, that in order to make a man or a boy covet a thing, it is only necessary to make the thing difficult to attain. If he had been a great and wise philosopher, like the writer of this book, he would now have comprehended that Work consists of whatever a body is obliged to do, and that Play consists of whatever a body is not obliged to do. And this would help him to understand why constructing artificial flowers or performing on a tread-mill is work, while rolling ten-pins or climbing Mont Blanc is only amusement. There are wealthy gentlemen in England who drive four-horse passenger-coaches twenty or thirty miles on a daily line, in the summer, because the privilege costs them considerable money; but if they were offered wages for the service, that would turn it into work and then they would resign.