The main line starts from the English town of Barrow-in-Furness. Crossing over the rolling lift bridge across the Walney Channel, the line calls at Vicarstown before passing through Henry's Tunnel. After this, the junction to the Norramby Branch Line is reached. The line passes a line after this, where Gordon's birdstrike incident happened. Also, in the television series, after passing through the Ballahoo tunnels and going past the junction to the Norramby branch line, there is a line that branches off the main line leading to Great Waterton which is nestled in between Stepney's Branch Line and the main line.
The line crosses River Hoo, and after that it calls at Crovan's Gate, home to the Works and the Skarloey Railway. After stopping here, the line crosses Ben Glas. Then it passes the place where Henry was stuck in the snow and was rescued by Donald and Douglas. Later comes the place where James' train brakes were mended with bootlaces. The line travels to Kellsthorpe Road, junction to the Kirk Ronan Branch Line. After that, it comes to the place where runaway James was caught by Edward. Crossing a bridge over a road, it comes to the stretch of line where Toby ran out of water. Later comes the bridge above Hawin Russagh. And the line afterwards reaches Killdane, junction to the Peel Godred Branch Line.
Cronk is next, and once leaving the station, trains cross a viaduct, and then arrive at Maron, junction to the Ulfstead Branch Line in the television series. Trains then descend Gordon's Hill and pull in at Wellsworth, the junction to Edward's Branch Line. The Cow Field is next, then trains arrive at Crosby, then the cow field where Edward's train was broken by some cows. Trains later pass through a mile-long tunnel, and then stop at Knapford, the junction to Thomas' Branch Line. Crossing another tunnel, the main line terminates at Tidmouth, home to the headquarters of the North Western Railway and the main line sheds. The junction to Duck's Branch Line is here as well.
The main line carries several kinds of freight, including ballast, lead, zinc, silver, slate, bauxite, produce, building stones and fish, including the Tidmouth kipper, which is considered a delicacy. Tourism is a major part of the passenger traffic during the summer season, as Sodor has many beautiful lakes, mountains, and other beautiful scenery. The main passenger traffic is commutering, with either stopping trains that stop at all main line stations or the "Wild Nor' Wester", the railway's express, which stops only at the Works station before reaching the Other Railway.
The line is staffed by Gordon, who usually pulls fast passenger trains, Henry and James, two mixed-traffic engines, and Bear (Railway series only), who assists with passenger duties. Donald and Douglas and the Works Diesel (Railway Series only) help out with shunting duties and goods trains. In the Railway Series, Pip and Emma take the "Wild Nor' Wester" from Sodor to London. When not working on the Brendam Branch Line, BoCo also helps out from time to time.
In the television series Emily and Murdoch also work on the line. Philip also works on the line at the shunting yards of Knapford. Even though Thomas primarily works on his branch line, following King of the Railway, Thomas has been known to pull "The Local" to Vicarstown on the Main Line with Annie and Clarabel. Philip also works on the line, but primarily at the shunting yards of Knapford. From the twenty-first series onwards, Rosie also works on the Main Line and is stationed at Vicarstown with Henry.