The landmark Canadian Pacific Railway bridge near Lethbridge is one of the largest railway structures in Canada. It was built in 1907-1909 as part of a major diversion of the Crowsnest Pass route between Lethbridge and Fort Macleod. It has been called either the Lethbridge Viaduct or the High Level Bridge at Lethbridge.

Originally, the Crowsnest railway mainline tracks actually bypassed Lethbridge which was reached by a one and a half mile (2.4 km) spur. The tracks swung south from Lethbridge, passed through Whoop-up and crossed the St. Mary's River on a long wooden trestle. Measuring 2,933 feet (894 m) long and 65 feet (20 m) high, the bridge was an impressive sight but steep grades were required to reach it.

Extensive cuts and many other bridges were needed to cross creeks and coulees (the wide steep-walled river valleys of the western Prairie rivers and streams). For example, after crossing the St Mary's River was passed several large cuts were required. One needed the excavation of 120,000 cubic yards (91 750 cubic metres) of materials. Some of the substantial bridges were a 900-foot (275-m) trestle with a 200-foot (61-m) span west of the St. Mary's River, a 600-foot (183-m) trestle at Eight-Mile Coulee and nearby another of 900 feet (274-m) in length and at Sixteen-Mile Coulee there was an 800-foot (244-m) trestle, 133 feet (40.5-m) high with a 200-foot (61-m) truss span over the creek. Overall, the trestles on the original line out of Lethbridge totalled 2.8 miles (4.5 km). Although the 37-mile (59.5-km) route accomplished its purpose in permitting the rapid completion of the railway, it proved to be an expensive section of railway to operate and one that was soon in need of replacement.

The original bridges built during the construction of the Crowsnest Route were designed to last only about 10 years and the CPR decided, after extensive surveys and engineering studies in 1904 and 1905, to construct a completely new route on a better gradient to bypass the original line. The new line was built to the north of the Old Man River and required two large bridges, one just west of Lethbridge - the famous High Level Bridge - over the Belly River and another over the Old Man River west of Monarch. It eliminated many curves and reduced the grade from 1.2 percent to only 0.4 percent and saved 5.26 miles (8.5 km) of track.

After engineering studies and planning, preliminary work began on construction in the summer of 1907. The first pile for the bridge was driven on October 26, 1907. Clearing, grading the site, the construction of piers and footings progressed while the steel work was being prefabricated. Once the steel work reached track level at the Lethbridge end, it was possible to begin using a huge travelling crane called an "erection traveller" which was used to lower the steel beams and girders into place. Raising the steel work began in mid-August 1908, the last girder was placed on June 20, 1909 and the riveting was completed on August 9, 1909. It took 645 railway cars to transport the steel to the bridge site and another 40 to bring in construction equipment. Overall, about 900 carloads of materials were needed for the project.

The massive structure over the Belly River is 5,327.625 feet (1 623.86 m) long and rose 314 feet (95.7 m) above the river bed. Over 12,400 tons of steel were used in its construction and the original cost was $1,334,525. The design incorporated 44 plate girder spans each 67 feet, 1 inch (20.4 m) in length and 22 plate girder spans each 98 feet, 10 inches (30.15 m) long. In addition, there was one riveted lattice-truss deck span 107 feet (32.6 m) long near the west end of the bridge. These spans were supported by a total of 33 rigidly braced steel towers. The second bridge on the new route, the Old Man River crossing, is 1,890 feet (576 m) long and 150 feet (45.7 m) high.

The Lethbridge Viaduct was designed by the CPR's bridge department in Montreal and the field work was directed by J. E. Schwitzer, Assistant Chief Engineer, CPR Western Lines. The steel work was manufactured by the Canadian Bridge Company of Walkerville, Ontario. C.F. Prettie was in charge of the bridge company's operations at the site and had a gang of about 100 men working under him. The bridge proved to be an enduring engineering work. Initially there were some problems with settlement of the underlying sediments around some of the footings but these were rectified. The big bridge is still in use.