Soils have limited load carrying capacity, which depends on their composition and water content. If this is exceeded by the transmitted loads the soil can no longer withstand the stress and gives way under plastic deformation. There are various ways in which the load carrying capacity of the soil can be improved. Originally reeds, straw and brushwood were used to improve soil properties, but today sand-gravel mixtures are preferred. Where their properties are not sufficient to give a satisfactory result, further action is needed. These range from profound building activities (e.g. bored piles, milling mixing-injection procedures, etc.) through hardening with binders which is used in road building to the use of geosynthetics as part of sand-gravel mixtures.
Far-reaching measures to improve the existing routes are only occasionally used in railways for special applications, because they often take a lot of time. The deliberate use of geosynthetics in sand-gravel mixtures improves their positive effect and allows the production with far more efficient construction methods. Originally, geosynthetics were used as pure non-woven fabrics. Their current tasks are versatile and include the following:
Separation, filtering, draining 
The filtering and separation task of the geotextile prevents mixing between the subgrade and the ballast. If there is exposed cohesive soil a passing train will trigger a pumping process, allowing fine particles to penetrate into the ballast bed. The use of geotextiles stops this effect, however, it does not seal the subgrade, but enables water to flow away. Water can seep away as usual or run into the side ditch.
Like the use of steel in reinforced concrete components, consideration was given to the use of geosynthetics to improve the load bearing capacity. Fabrics and geogrids can accept for this purpose large tensile forces under high shape stability. The low change of shape and the large contact surface to the surrounding material are necessary in order to distribute the traffic loads better on the surrounding soil. Geogrids fulfil these tasks very well due to their specific structure. A combination of these properties enables composites to be used. These are made from geotextile components and geogrids and thus combine the properties of both groups. 
Multi-layered "reinforced" track bed layer systems can be installed on-track and the load bearing capacity significantly improved. The operating limitations and the costs can be significantly reduced in comparison to far-reaching measures. This possibility is, therefore, a worthwhile alternative, especially on main lines. 
In some secondary lines, geotextiles are used directly in the ballast in order to increase the load carrying capacity of the system, but this makes the future upgrading more difficult. Such restrictions should, therefore, already be taken into account during Installation.
Especially in water protection areas the adjacent soil must be protected from possible seepage of contaminated surface water. The water is collected and fed to the appropriate recipient by the shortest path. In order to prevent seepage, the subgrade is covered with a layer that is impermeable to water over the desired area.
Mineral seals, plastic membranes and geosynthetic clay sealing lines can be used for this purpose. During installation, attention should be given to the different special requirements. Mineral seals from cohesive soils are not recommended because of their sensitivity to frost and low deformation resistance. 
Geosynthetics with protection tasks keep sensitive components (e.g. sealing membranes) against mechanical damage. This particularly applies to shear and pressure stress caused by the bordering soil. Geosynthetics with a protective function must have sufficient thickness and a high resistance to perforation.
In addition to the traditional use of geosynthetics in the direct area of the track, these are also used in conventional earth and support structures as well as side path constructions. 
-  Fischer, R.; Göbel, C.; Lieberenz, K. et al.: Handbuch Erdbauwerke der Bahnen. Planung, Bemessung, Ausführung, Instandhaltung. Eurailpress in DVV Media Group, Hamburg, 2013.
-  Hempel, M.; Hangen, H.: Einsatz von Geokunststoffen im Eisenbahnbau. Der Eisenbahningenieur 2016 (2016), Sonderheft Geotechnik, S. 4–6.