Complex sentence: The trail is part of a rails to trails project between Pittsburgh and Cumberland, Maryland that makes up part of a trail system between Pittsburgh and Washington, D.C, known as the Great Allegheny Passage.
Split into simpler sentences: The trail is part of a rails to trails project between Pittsburgh and Cumberland, Maryland. When completed, it will be part of a trail system between Pittsburgh and Washington, D.C, known as the Great Allegheny Passage.