In August 1905 the NEC set up the Musselburgh and District Electric Light and Traction Company to operate the line, and the system was expanded with an extension to Tranent Road in Cockenzie opening on 5 August 1909, and on 31 December 1909 to Port Seton bringing the total single line route to 6.55 miles.

Due to narrow streets, the line was mostly single track, with passing loops at various places.

[2] Construction of the Port Seton Golf Club course was largely funded by the company, as their terminus was midway along the proposed course.

It opened in 1912, utilising 100 acres of land leased from Francis Charteris, 10th Earl of Wemyss.

[4] Near Morrison's Haven, a section of one double-track loop about 58m in length and laid in setts survives, and was uncovered at some point for educational purposes.