As a part of the European research project INNOTRACK (INNOvative TRACK systems) which aimed at reducing the Life Cycle Cost (LCC) of railway infrastructure, an original research work was undertaken in collaboration between two companies (Keller Foundations and Solétanche Bachy), SNCF (French National Railway Company), and LCPC (French Public Works Research Laboratory). A field test was carried out on a railtrack of the high-speed line in Northern France. The main objective of this work was to show the feasibility of a technique of reinforcement of the subgrade of existing tracks. The method consists in building vertical soil-cement columns under the sub-ballast layer without removing the track. This technique limits the duration of maintenance work and, consequently, traffic interruption. In addition, it makes use of the existing soil, reduces the amount of concrete and cement needed, and does not create stiff zones at the location of each column that might accelerate the degradation of ballast. Finite element simulations were performed to improve our understanding of the mechanical response of the system track-columns and to optimize the design.