Leepers made from old bottles, food packaging and other unwanted plastics were first installed on UK railroads.
Network Rail said using recycled materials in Wiltshire would help it achieve its ambition of achieving net zero carbon emissions by 2050.
Sleepers, which hold the rails and keep them at the correct distance, are normally made of concrete or wood.
The composite version will have a longer lifespan and reduced maintenance, Network Rail said.
They are designed to be used for 50 years because they will not split, rot or degrade, and can resist water, oil, chemicals and fungus.
Rail Minister Chris Heaton-Harris said: “I am proud to see such a positive innovation being used for the first time on the mainline.
“Not only are these ties made from locally sourced plastic waste, they require less maintenance and will last longer, underscoring our commitment to creating a greener, cleaner and more efficient rail network.
The ties were recently installed on the Sherrington Viaduct, between Salisbury and Warminster.
Mark Killick, Network Rail’s Wessex Line Manager, commented: “This is an exciting development. The use of these recycled ties on the Network Rail Wessex route is a first for the overhead rail network in Britain.
“Rail is already one of the most environmentally friendly forms of transportation, but we are committed to delivering even greener and better journeys, whether it’s changing the way we maintain the wayside or find innovative ways to improve the railway by reusing materials and reducing landfill.
“By using these sleepers, not only are we improving the track for customers, but they will travel on a railway line built with sustainable materials as part of the circular economy. “