Complex sentence: Horsepower for naturally aspirated (Roots-blown) engines is usually derated 2.5 percent per above mean sea level, a tremendous penalty at the or greater elevations which several Western U.S. and Canada railroads operate, and this can amount to a 25 percent power loss.
Split into simpler sentences: Horsepower for naturally aspirated (Roots-blown) engines is usually derated 2.5 percent per above mean sea level, a tremendous penalty. At the or greater elevations which several Western U.S. and Canada railroads operate this can amount to a 25 percent power loss.