Complex sentence: The first section comprises the "Fréjus" tunnel, and its lowest tunnel ceilings, highest elevation, sharpest curves, and steepest gradients, make it the limiting factor on the overall capacity of the line.
Split into simpler sentences: The first section comprises the "Fréjus" tunnel. Its lowest tunnel ceilings, highest elevation, sharpest curves, and steepest gradients, make it the limiting factor on the overall capacity of the line.