Points on the railway line

“Points,” he muttered, “the points.”
“What of it? What do you mean?”
“I suppose there are no great number of points on a system such as this?”
“No; there are very few.”
“And a curve, too. Points, and a curve."
"Is it a coincidence that it is found at the very point where the train pitches and sways as it comes round on the points? Is not that the place where an object upon the roof might be expected to fall off? The points would affect no object inside the train." (Sherlock Holmes)
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