What was the most important trade route through the Appalachian mountains in the mid-1800s?

Answer 1

The Erie Canal, first proposed in 1807, was built between 1817 and 1825 to connect the Atlantic Ocean to the Great Lakes by way of the Hudson River. The canal connected Albany and Buffalo, stretching for over 363 miles (584 km).

Answer 2

Date wise, the routes would be:

• 1755 Braddock's Road (Pennsylvania)
  
• 1758 Forbes Road (Pennsylvania)
  
• 1700s Cumberland Gap - oldest in terms of a route provided by nature and geography, but as a southern route it's use dates closer to Braddock and Forbes roads. (Pennsylvania) because settlers were forbidden to move west until the mid 1700s. Treaties with Native Americans kept whites on the East side of the mountains.
  
• 1700s - oldest river routes - Ohio River north to The Allegheny River and Monongahela River to Pittsburgh PA and Ohio Territory which opened to settlers late 1790s; OR, Ohio River southwesterly to the Mississippi River to points north OR points south to the Gulf of Mexico
  
• 1811 - 1837 - The Cumberland Road ( National Road ; e.g. Route 40 E and W) - (Pennsylvania)
  
• 1817 - 1825 The Erie Canal across New York
  

From North to South the Routes are:

From Northernmost areas, people used the Erie Canal from the Hudson River route. See second answer below.

In the centermost to south-center section of Pennsylvania, Braddock's Road (Braddock Road or Braddock expedition, aka Braddock's trail) was a military road built in 1755. It crossed the successive ridges of the Appalachian Mountains in Pennsylvania by land. It's original path followed the Indian footpaths along the ridges.

The Forbes Expedition built Forbes Road in 1758. It took a slightly different route.

The Cumberland Road which became known as the National Road and later, U.S. Route 40, roughly follows the route of Braddock's Road between Cumberland, Maryland, and Laurel Ridge near Uniontown Pennsylvania.

1700s - oldest river routes - Ohio River north to The Allegheny River and Monongahela River to Pittsburgh PA and Ohio; OR, Ohio River southwesterly to the Mississippi River to points north OR points south to the Gulf of Mexico

The Cumberland Gap is the lowest point as a narrow pass through the Appalachian Mountains by land. Cumberland Gap is in the southeast portion of the Mountains at 1,631 ft (497 m) elevation closest to Virginia, Tennessee, and Kentucky. This means from NJ, PA, MD, VA traders took a southeastern route over land to the Gap.