The curve was a mid-19th-century solution to the problem of how to get a railroad over the Allegheny Mountains. It was the engineering marvel of its time, the three years' work of 450 Irish laborers who, using nothing but picks, shovels, and black powder, carved a ledge onto the side of a mountain and then laid 2,375 feet of track along a steep grade.
Even now, it inspires the wonder of rail fans to see both the front end and back end of a long train as it grinds and screeches around the perfect U-shape.
"Which way?" Joanne Brown says of the coming train.
"From the strain, I'd say uphill," Sam Brown replies.
The leaves of the heavily forested Alleghenies, which sit in lumps across the horizon, have started to turn. A reservoir for the nearby city of Altoona lies below, in the cupped hands of the surrounding hills.
The low moan becomes a rumble as the twin diesel locomotives of a Norfolk southern train come around the base of the curve.
The rumble becomes a roar as the black diesels work their way upward, pulling flat cars, each stacked with two truck trailers, one atop the other.
The roar becomes thunder as the train reaches the main viewing area of the Horseshoe Curve National Historic Landmark, where an inclined railroad was built in 1992 to ferry visitors up a 122-foot cliff to gain a close-up look.
The train buffs crowd the fence. The engineer waves.
"I don't know what it is, but I have loved trains since I was a kid," Joanne Brown yells above the noise.
"The railroad people call us foamers," Sam Brown says.
"Because we stand by the tracks and foam at the mouth," Joanne laughs.
"We have laid down on the rails and taken pictures," Joanne says. "We have hung off bridges."
Each day, about 60 trains pass through the curve. You can sit for an hour and see three or four freights. You have to have luck, or know the schedule, to see a passenger train on what used to be the main line of the Pennsylvania Railroad. There's an eastbound Amtrak at 9:30 a.m. and a westbound Amtrak at 5:30 p.m.
E. Steven Barry, editor of Railfan & Railroad Magazine, says from his office in Newton, N.J., that the Horseshoe Curve is "one of the top 10 holy sites of rail-fanning in the United States."
"It is still one of the busiest spots for freight traffic in the East," he says. "The combination of the curve wrapping around you, coupled with the steep grade there, makes for a lot of noise and dramatic action."
"That whole area," he said, "has developed a cottage industry of mom-and-pop railroad attractions."
The historic site itself, which has a peaked-roof museum and gift shop in the style of an old station, is administered by the Railroaders Memorial Museum in Altoona.
The Pennsy, in its heyday, employed thousands of people at its Altoona shops.
"My grandfather worked for the Pennsylvania Railroad and all his sons worked for the Pennsylvania Railroad," says Colin Knorr, who was visiting the curve with his wife, Pat.
Knorr, a retired trucker whose roots are in Altoona, says there is something hypnotic about watching trains all day. "You can sit back and relax, and look around."
John and Ernestine Hamilton of Mill Hall, Pa., got engaged to be married at the Horseshoe Curve 33 years ago.
Wouldn't Ernestine have preferred a fancy restaurant?
"Maybe," she says. "But I like it up here."
"It's peaceful," John says.
Indeed, between trains, the only sound is the leaves in the trees. At this altitude - 1,505 feet at the east end of the curve, 1,716 feet at the west end - many of the trees are showing red already.
"Oh, I hear another train," says Carol Cassidy, a visitor from Kensington, Conn.
"I was thinking I hear one, too," says her husband, Ray.
The Cassidys each lost their first spouse. They are recently married. He's a retired electronic engineer; she's a retired office manager.
Her first husband loved to fish, Carol Cassidy says. "I am getting used to all this train stuff now."
Ray Cassidy makes no apologies for his obsession - or for dragging his bride around to rail sites.
"When I was real young, growing up, from my bedroom window I could look down the hill and there was a freight train that would go by every morning. And every afternoon, it would go the other way."
"I loved that."
Contact Tom Infield at 610-313-8205 or email@example.com, or follow on Twitter @tinfield.