Learn Spanish (Latin America): Los vagones son rapidísimos porque alcanzan mucha velocidad. - The cars are extremely fast because they reach a lot of speed.

The base adjective is rápido. To form the absolute superlative (meaning extremely fast or super fast), you drop the final vowel -o and add the suffix -ísimo. We use the masculine plural ending -os (rapidísimos) to match the masculine plural noun it describes: los vagones. Notice that the written accent mark stays on the same vowel as the original word!

Why use vagones for cars instead of autos or carros?

Vagones specifically refers to train cars, subway cars, or rollercoaster cars. If you were talking about standard passenger automobiles on a street or highway, you would use autos or carros.

Why is it mucha velocidad and not mucho velocidad?

In Spanish, nouns ending in -dad (like velocidad, ciudad, and verdad) are almost always feminine. Therefore, the adjective mucho has to change to its feminine form, mucha, to match it.

Does the verb alcanzan only mean to reach a certain speed?