Complex sentence: The highway engineers did not use most of the railroad's other grading, however, since they could afford steeper grades and shorter alignments--so relics of the "ghost railroad" may still be found all across the Alleghenies.
Split into simpler sentences: The highway engineers did not use most of the railroad's other grading, however, since they could afford steeper grades and shorter alignments. Because of this, relics of the "ghost railroad" may still be found all across the Alleghenies.