Brockville Ontario - Canada's longest railway tunnel dating from the 1850s, why did the railway company build the railway under downtown Brockville to the lake through bedrock?
 
I can't imagine the town was heavily built up at the time, and it would have been a lot easier and cheaper to build the line above ground.
 
I didn't see an explanation at the site on why they decided to build a 500 metre tunnel instead.
 
Unknown person.
 

"Why Was the Brockville Tunnel Built Underground?" Because they don't build tunnels above ground !  Here's the answer from the Brockville Tunnel page...

Although little known, the oldest railway tunnel in Canada actually exists under the downtown core of Brockville, Ontario.
 
The tunnel was built between 1854 and 1860 to allow the fledging Brockville & Ottawa Railway to connect the Brockville industrial waterfront area to the outlying areas lying between the St Lawrence and Ottawa rivers.
 
Samuel Keefer, who at age 30 had been appointed to the highest engineering position in Canada, the first chief engineer of the Board of Public Works of the United Provinces, was opposed to the tunnel project.
 
He recommended the rail line run around the high grade of the community and simply come down to the river from a westerly direction thus avoiding the hill.
 
However, the philosophy of the Canadian railway builders seemed to be based on the belief that no railway was complete without at least one tunnel.
 
The council of the day and the citizens of Brockville seemed to agree and pledged their support to the building of the tunnel under the city for a direct access north.
 
Author unknown.