The Lethbridge viaduct.
1909 - 1,624m long
Longest and highest trestle and there are no weight or length restrictions so toughest too. The CP engineer responsible also was responsible for the Spiral Tunnels.
http://dailyphotodose.com/page/2/?s=highlevel+bridge
Photo: Jaime Vedres
Not human engineering but...
The world's largest beaver damn in Wood Buffalo National Park AB. 850m long.
http://www.geostrategis.com/p_beavers-longestdam.htm