I've read that chains don't stretch. The holes for the pins become elongated through wear. Chains made to shift between sprocket wheels flex sideways, so how can one tell the difference?
In 1965 I had an experience where a bicycle chain didn't stretch operating under an estimated 432 pounds of tension. I owned an old A B Jackson, which was a Raleigh smuggled in a box classified as motorcycle parts. It had internal shift: 93, 70, and 52 gear inches. (The tallest of my Abound's 7 gears is 73 gear inches.) I lived at the bottom of a 10% grade 1,000 feet long. I guess I weighed 170 and the bike 30, so that hill required 20 pounds of thrust just to match gravity. I'd end up in low gear, where I needed a mean pedal force of 75 pounds and, as a rule of thumb, 112 pounds peak, just to meet gravity. I hated low gear because it was so slow.
One evening I got impatient. I stood up, shifted to high, and by pulling up hard on the bars like a weightlifter, put more than my weight on each power stroke. To my amazement, I accelerated sharply at 93 inch pounds on a 10% hill. My exhilaration was brief. In 5 seconds I'd torn the teeth from the rear sprocket.
If I sped up 5 mph in those 5 seconds, that would have required 9 pounds of thrust in addition to 20 pounds to match gravity. That would mean an average pedal pressure of 192 pounds and a peak of 288. Strain on a chain is about 75% higher than force on a pedal, or 432 pounds peak in this case. That explained how I'd torn the teeth off the sprocket wheel, which hadn't shown wear.
Under 432 pounds of tension, had the holes for the pins elongated? No. A chain for bikes without derailleurs was designed not to flex laterally. Like a new chainsaw chain, this one still had no flex. Had the rails stretched? No. The front sprocket had no apparent wear, and the chain still fit it perfectly. It also fit the new rear sprocket perfectly.