After watching a number of videos of high-speed trains in Europe and comparing them to what I saw of Amtrak's Acela Express, I became convinced that the Acela trains were significantly shorter than TGV trainsets. Our train has one power car at each end plus six intermediate cars, while the TGV has eight intermediate cars plus the two power cars.
However, it turns out that TGV and Acela trainsets are very similar in length. The Acela has carriages about 87 feet 5 inches (26.6 m) long, while TGV carriages extend about 65 feet (20 m). Power cars seem to be about the same size here as they are across the pond. The end result is that the Acela is about seven feet longer than the TGV.
Listening to the whooshing noises as the trains passed by in video clips enhanced the sense of speed from the TGVs: At 186 miles per hour, about 4.2 carriages pass by each second. An Acela running at the current top speed of 150 mph will only have 2.5 carriages pass in the same time. It looks and sounds like the American train is going 40% slower, but the actual difference in speed is only about 20%.
Of course, TGV trainsets are frequently coupled together to form trains that are about a quarter mile (400 m) long, so they are still longer overall in many cases. Similarly, Eurostar trains that run from London to Paris are built from the ground up at this larger size. Japan's Shinkansen can also extend about a quarter mile. Trains of this length aren't entirely unheard-of in the U.S., however—the Empire Builder often runs pretty close to that long. You're not going to catch the EB going anywhere close to 150 or 186 mph anytime soon, but considering that Superliner cars are about 85 feet long, your viewing pleasure may be usurped by a similar effect.