Network Rail services have returned to normal after the 10-day shutdown over the festive period for improvement works.
The service used the time to close the line between Ingatestone and Liverpool Street to replace the overhead wires and some track on the route.
The work was part of a £250m project to install a new wiring system that automatically adapts to temperature changes to reduce delays.
The work meant there was no Transport for London service between Shenfield and Stratford over the Christmas period.
Route managing director Meliha Duymaz said: “Following the installation of overhead wire, we will no longer need to impose speed restrictions between Chelmsford and Ilford in hot weather, which means fewer delays and cancellations.
“Further work will take place at weekends from the end of January and I would like to thank passengers for their patience while we carry out these important upgrades.”
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