This leaflet relates to earthworks and track beds for ballasted track. The study of railway track substructure can be considered as two separate subjects: the design of track bed layers and earthworks for new lines; maintenance of the track bed layers and earthworks on existing lines, which show signs of ageing or are exposed to a more severe loading regime (increase in gross tonnage, axle-load or speed). The technical design life considered in this leaflet for substructure is of 100 years. The leaflet is divided into 3 sections: terminology and classification of soils and subgrades, construction of earthworks and track bed layers for new lines, maintenance of earthworks and track bed layers on existing lines.