The 5.022-mile (8.082 km) tunnel carries the Canadian Pacific Kansas City (CPKC) main line under Mount Macdonald in the Selkirk Mountains.

The tunnel, opened by the Canadian Pacific Railway (CP) in 1916, replaced the previous routing over Rogers Pass.

Traffic restrictions imposed by a single track comprising 46 miles (74 km) of 2.2 percent gradients,[1] emerging competition,[2] and snow-related costs,[3] were negative factors.

[10] The opening of the Grand Trunk Pacific (GTP) posed a threat to CP's grain and Asian trade.

[12] The opening of the Canadian Northern Railway (CNoR) via the Yellowhead to Vancouver, the premier destination, prompted greater alarm.

This alignment would have eliminated the bridges at Mountain, Surprise, and Stoney creeks, but was rejected owing to perceived construction deadlines, and a route beneath the headwaters of the Illecillewaet River, vulnerable to leakage.

[23] John G. Sullivan, chief engineer,[24] recommended a 5.3-mile (8.5 km) tunnel, similar to, but more expensive than, the Busteed one.

Primarily, the tunnel lowered the grade; secondly, it shortened the distance; and thirdly, it bypassed an avalanche-prone zone.

After negotiations with the lowest bidder, the contract was awarded to Foley, Welch and Stewart (FW&S) in July 1913.

Both comprised a police post, small hospital, general store, offices, apartments, bunkhouses, kitchen, dining hall, and lounge, with electric lighting and plumbing for water and sanitation.

[32] Beating world monthly tunneling records, the pioneer headings met in December 1915, and the main bores in July 1916.

A tunnel watchman, who rushed some distance to the nearest telephone to alert Connaught station at the eastern portal, did not survive.

[48] In 1929, two train crew died when their locomotive fell into the ravine when a girder collapsed on the Surprise Creek Bridge.

[49] The 1931 flood sent 2,000 replacement ties through the tunnel and filled the west portal cutting to a 20-foot (6.1 m) depth[50] with 10,000 to 12,000 cubic yards (7,600 to 9,200 m3) of mud and debris, which took five days to dig out.

[38] In 1977, soon after departing the Glacier siding (Mile 85.9), a westbound loaded 109-car coal train lost control, and broadcast a warning that they were travelling too fast to make the curves at Illecillewaet.

On hearing the radio message, a 60-car eastbound freight accelerated to safely reach Illecillewaet siding (Mile 98.1), before three crew members stepped clear of their train.

[55] The first commercial train was December 1916, which traveled via Loop Spur, since the pass line remained in use until a few days later.

[56] The most northern part of the Loops between the two hillsides, which had been a long trestle,[57] but likely infilled around 1906, needed to be breached by the new line.

[58] To improve visibility on the 1929 introduction of the Selkirk locomotives, trains switched to the left-hand track before entering the tunnel.

On occasions, when pushers worked through to Glacier, crew were required to wear respirators, owing to the tunnel fumes.

After the 1988 opening of the Mount Macdonald Tunnel, westbound traffic primarily used that lower gradient route, with Connaught handling eastbound.

A. McIlwee and Sons encountered an unexpected predominance of crumbly slate, FW&S dismissed the subcontractor, rather than negotiate a contract variance.

After several appeals, including a hearing of the Judicial Committee of the Privy Council, the courts awarded McIlwee about $576,000 for breach of contract.