Sufficient load-bearing capacity of the subsoil is the basis for the sustainable and safe operation of a railway system. Even at the beginning of the 20th century the negative impact on the track of too little load bearing capacity and standing water was recognised. The expression, "As you make your bed so you must lie on it." might be the most fitting description of the relation between subgrade and track. Subgrade problems are often recognised visually very late. The properties of the overall system change over the years. Their effects are often irreversible, leading to massive maintenance required, and finally to the early replacement of the entire track.
When track is renewed (conversion) or new sections of track are laid on the route, in particular with regard to the introduction of concrete sleepers, the need for formation rehabilitation should be checked. This can be done without track in position or with track in position. Near-surface measures, such as the introduction of Geosynthetics and Sand-gravel mixtures can be carried out with track in position and without track in position. Far-reaching measures, such as vibro-columns, can only be done when the track is removed.
Formation rehabilitation without track present
On existing lines the first step is the removal of the existing track skeleton. This can be done both with track in position by the Relaying trains or without track in position after the track panels have been removed. After this there is:
- The removal of the part of the bed which is not able to support sufficient load.
- Manufacture and consolidation of the formation.
- Installation of the load carrying and protective layer with the possible use of geosynthetics.
- Accompanying consolidation of the protective layers.
- Placement of ballast.
- Assembly of the track skeleton.
- Restoration of the track geometry through repeated tamping cycles and new ballast supply.
Formation rehabilitations after the track has been removed are carried out with the usual earthmoving equipment and in comparison with the on-track procedure show significantly lower costs. Due to the reduced working speed the working track is out of service for a longer period of time. The working area has to be particularly protected for occupational safety reasons from the adjacent track (if present). When using bucket excavators care should be taken to limit the swivel range, in order not to endanger the operation on the adjacent track. Especially in the case of short worksites and strongly varying ground conditions the procedure without track in position shows its advantages. Due to the large number of pieces of construction equipment used (e.g. chain excavators, bulldozers, graders, chain dumpers etc.) attention must be paid to documentation of the quality of execution. A qualified construction site supervisor is essential for this process.
On-track formation rehabilitation
In the track-bound procedure it is not necessary to dismantle the existing railway system. Specially designed large machines use the track, in order to take off all construction processes, as well as the material disposition in the framework of the working track. The railway can continue to operate on the adjacent line, where appropriate, with a speed restriction. Starting with the Plasser & Theurer PM 200, numerous machines have been developed since 1983, to meet the specific national requirements of the formation improvement of different railway Infrastructure managers.
In 1994 came the AHM 800 R and so for the first time a machine was put into service which removed the existing ballast, prepared it, and installed it again as part of the protective layer. The ballast is broken by the integrated cone crusher to a particle size of 0 to 35 mm, supplemented with new material, and put in again as a protective layer. Track-bound methods allow the re-use of the removed material and thus reduce the amount of new material necessary. Despite their prodigious size, these machines are very flexible with regard to the processing possibilities. The different methods of on-track formation rehabilitation can be divided into the following groups:
Procedure with processing of the excavated material as a protective layer
Procedure with processing of the material as track ballast
Combined track formation improvement and ballast cleaning machines
Procedure without material preparation
Regardless of machine-dependent characteristics there are the following standard processes:
- Lifting the track skeleton.
- Inserting the excavating chain.
- Removing the track ballast and the substructure (partly).
- Producing the elevation, levelness and cross fall of the formation.
- Relocating the track bed layer system and possible installation of geosynthetics.
- Grading and consolidation of the protective layer.
- Placing the track skeleton on the formation which has been supplemented by new ballast and ballast that has been brought back again.
- Restoring the track geometry through repeated tamping cycles and new ballast supply.
Current studies show that rail mounted procedures are at least equal to procedures carried out without track in position with regard to homogeneity and quality.  The table below attempts to summarise the advantages and disadvantages of both methods.
You can find suitable specialist literature to the topic here:
The Basic Principles of Mechanised Track Maintenance
This book is dedicated to the many people involved in the day to day planning and performance of track maintenance activities. Providing a practical approach to everyday challenges in mechanised track maintenance, it is not just intended as a theoretical approach to the track system.
Railways aim at transporting people and freight safely, rapidly, regularly, comfortably and on time from one place to another. This book is directed to track infrastructure departments contributing to the above objective by ensuring the track infrastructure’s reliability, availability, maintainability and safety – denoted by the acronym RAMS. Regular, effective and affordable track maintenance enable RAMS to be achieved.
-  Hansmann, F.: Großmaschineneinsätze bei Untergründen mit geringer Tragfähigkeit, Sonderheft Geotechnik. Der Eisenbahningenieur 2016, Heft 6, S. 13–16.
-  Wenty, R.; Schreiner, H.: Rehabbing subgrade with track in place, in: Railway Track & Structures, 2005
-  Auer, F.; Zuzic, M.; Schilder, R.; Breymann, H.: 13 Jahre Erfahrung mit der gleisgebundenen Untergrundsanierung im Netz der ÖBB, in: ETR, Eisenbahntechnische Rundschau, 2007, S. 817
-  Veit, P.: Sanierung, Instandsetzung und Neubau im Lebenszyklus, iaf Kongress BahnBau, 2011