Complex sentence: It remains mainly intact to this date as the stretch of Route 4 from U.S. Route 1 to Routes 2 and 102 at Exit 5; the only piece of this arterial that has been significantly altered is the construction of a bridge over Amtrak's Northeast Corridor line.
Split into simpler sentences: It remains mainly intact to this date as the stretch of Route 4 from U.S. Route 1 to Routes 102 and 2 at Exit 5. The only piece of this arterial that has been significantly changed is the construction of an overpass for Amtrak's Northeast Corridor line.