Rail tracks are conventionally built on compacted ballast and structural fill embankments overlying the natural subsoil. Ballast plays an important role in providing track stiffness to support heavy traffic loads, and providing rapid drainage. However, ballast deforms and degrades progressively under the heavy cyclic loading of passenger and freight trains, which may lead to a loss of track geometry, and require costly regular maintenance. In particular, track construction requires appropriate stabilization techniques for ballast, the extent of which depends also on the type of subgrade. Comprehensive field trials were carried out on two rail lines in Bulli and recently in Singleton, New South Wales, Australia. In these studies, several track sections were reinforced with different types of geosynthetics placed beneath the ballast embankment. Both fresh and recycled ballast was examined for varying subgrade conditions. Recoverable and irrecoverable deformations of the substructure were routinely monitored. It was found that geogrids and geocomposites can decrease the vertical strains of the ballast layer, resulting in reduced maintenance costs. This paper describes the comprehensive field instrumentation, construction procedures, and field performance evaluation of these full-scale geosynthetic- stabilized ballast embankments in Bulli and Singleton.