What obstacle stood in the way of the Baltimore-Ohio railroad

Appalachian Mountains were considered to be the great barrier in building the Baltimore and Ohio road.

Explanation:

The key element for the growth of US economy was that its expansion in trade towards the west. Baltimore was considered to be a great commercial place because it was connected to National Road. But transportation of freight through this road seems to be expensive and hence the need for another rail road was felt which paved way for the construction of Baltimore Ohio Road.

After this project, Baltimore underwent a series of changes in transportation, freight and how people and ideas traveled across the country. From then on, there was a transition in preferences which is from road to rails.