question: what keeps train tracks grounded to the floor?
comment: The weight of any train going over them pushes down on them, in addition to their own weight and the static friction coefficient between sleepers and ground is higher than the rolling or sliding friction coefficient between train and track. Combined with the structural strength of the tracks this means they won't be moved significantly by the train under normal operation. If the train gets pushed sideways, they do move.
passages: Moisture from exhausts and springs can also make the rails slippery, and allowance may need to be taken for that as well.
  
  Section::::General situation in the United States.
   Spikes in wooden ties can loosen over time, but split and rotten ties may be individually replaced with new wooden ties or concrete substitutes. Concrete ties can also develop cracks or splits, and can also be replaced individually. Should the rails settle due to soil subsidence, they can be lifted by specialized machinery and additional ballast tamped under the ties to level the rails.
   Section::::Ground vibrations from railways.
   Insulated blockjoints can be bridged by wheel scale in some circumstances causing one or two track circuits to fail. This problem may be reduced by having a pair of blockjoints in series about 4m apart. The short 4m section would not itself be track circuited.
  
  Section::::Failure modes and prevention.:Immunization.
  
  Electric locomotives must avoid generating noise in the frequencies that track circuits use. The SNCB Class 13 had such problems.
  
  Section::::Failure modes and prevention.:Transitory problems.
   The trains have metal contact blocks called collector shoes (or contact shoes or pickup shoes) which make contact with the conductor rail. The traction current is returned to the generating station through the running rails. In the US, the conductor rail is usually made of high conductivity steel or steel bolted to aluminium to increase the conductivity. Elsewhere in the world, extruded aluminum conductors with stainless steel contact surface or cap, is the preferred technology due to its lower electrical resistance, longer life, and lighter weight. The running rails are electrically connected using wire bonds or other devices, to minimise resistance in the electric circuit. Contact shoes can be positioned below, above, or beside the third rail, depending on the type of third rail used: these third rails are referred to as bottom-contact, top-contact, or side-contact, respectively.
   Various methods exist for fixing the rail to the sleeper. Historically spikes gave way to cast iron chairs fixed to the sleeper, more recently springs (such as "Pandrol clips") are used to fix the rail to the sleeper chair.
  
  Section::::Portable track.
   These systems are initially calibrated to a particular ‘neutral state’ but measuring the rail with neutral compression (no pulling or pushing) and an agreed neutral ambient temperature. The system then measures the rail to detect if these parameters get too far from neutral state, and will alert if the rail is approaching a breach of structural integrity.
  
  Section::::Sensors.:Ground Based Noise (GBN).
   A track circuit typically has power applied to each rail and a relay coil wired across them. When no train is present, the relay is energised by the current flowing from the power source through the rails. When a train is present, its axles short (shunt) the rails together; the current to the track relay coil drops, and it is de-energised. Circuits through the relay contacts therefore report whether or not the track is occupied.
   Another unusual aspect was the configuration of the third rail. Unlike other American commuter-rail agencies that operate trains powered by third rails, which have a contact shoe on top of their third rail, Metro-North trains' contact shoes draw current from the bottom of the third rail during operation. The railroad's under-running third rails are designed in order to prevent ice from building up on top during winter months and reduce the possibility of inadvertent contact with the high-voltage rail; as a consequence, they are much safer than the traditional over-running third rails. To facilitate this, the ends of the third rails adjacent to grade crossings have a slight upturn. 
   Periodically, ballast must be removed and replaced with clean ballast to ensure adequate drainage. Culverts and other passages for water must be kept clear lest water is impounded by the trackbed, causing landslips. Where trackbeds are placed along rivers, additional protection is usually placed to prevent streambank erosion during times of high water. Bridges require inspection and maintenance, since they are subject to large surges of stress in a short period of time when a heavy train crosses.
  
  Section::::Infrastructure.:Train inspection systems.
   A traditional track structure consists of two rails, fixed at a designated distance apart (known as the track gauge), and supported on transverse sleepers (ties). Some advanced track structures support the rails on a concrete or asphalt slab. The running surface of the rails is required to be practically continuous and of the proper geometrical layout.
   The track circuit relies upon an adequate electrical contact between the rail and the wheel; contamination can insulate the one from the other. A common problem is fallen leaves, though there have been cases where crushed insects have also caused detection failures.
  
  A more persistent problem is rust. Usually the railhead is kept clean of rust by the regular passage of trains' wheels. Lines which are not used regularly can become so rusty as to prevent vehicles being detected; seldom-used points and crossovers and the extremities of terminal platform lines are also prone to rusting. Measures to overcome this include:
   Much rarer are accidents caused when the track circuits themselves fail. For example:
  
  BULLET::::- Cowan rail disaster, which occurred when sand on the rails insulated the wheels from the rails, causing a failure to shunt that allowed a trailing block signal to improperly display a clear aspect, resulting in a rear-end collision.
  
  BULLET::::- June 22, 2009 Washington Metro train collision, which occurred when a parasitic oscillation developed in a track circuit control module.
  
  Section::::Accidents.:Broken rails.
    Because of the gaps in the conductor rail (e.g., at level crossings and junctions) a train can stop in a position where all of its power pickup shoes are in gaps, so that no traction power is available. The train is then said to be "gapped". Another train must then be brought up behind the stranded train to push it on to the conductor rail, or a jumper cable may be used to supply enough power to the train to get one of its contact shoes back on the third rail. Avoiding this problem requires a minimum length of trains that can be run on a line. Locomotives have either had the backup of an on-board diesel engine system (e.g., British Rail Class 73), or have been connected to shoes on the rolling stock (e.g. Metropolitan Railway).
   Usually, the train floor is at a level above the bogies, but the floor of the car may be lower between bogies, such as for a bilevel rail car to increase interior space while staying within height restrictions, or in easy-access, stepless-entry, low-floor trains.
  
  Section::::Railway.:Components.
  
  Key components of a bogie include:
  
  BULLET::::- The bogie frame: This can be of inside frame type where the main frame and bearings are between the wheels, or (more commonly) of outside frame type where the main frame and bearings are outside the wheels.
   The natural, pre-installation length of the conductor rail is . During installation, the contact rail segments are welded together to produce conductor rails of varying length. In curved sections with a radius of or more, straightaways, and tunnels, the contact rail is welded to a length of ; at surface running, ; and, on tight curves and park paths, .
  
  Post-Soviet third rail installations use the bottom-contact (Wilgus-Sprague) system; on top of the rail is high strength plastic casing with sufficient structural integrity to support a man's weight. Voltage is 825 volts DC.
  
  Section::::Mixed systems.:United States.
   It is important to mention that the occupancy calculated in these systems must include a safety margin for location uncertainty (in yellow in the figure) added to the length of the train. Both of them form what is usually called 'Footprint'. This safety margin depends on the accuracy of the odometry system in the train.
   Section::::Infrastructure.:Tunnels, viaducts and bridges.
  
  Major civil engineering structures on the East Coast Main Line include
  
  Section::::Infrastructure.:Line-side monitoring equipment.
  
  Line-side train monitoring equipment includes hot axle box detectors (HABD) and wheel impact load detectors (WILD) ‘Wheelchex’, these are located as follows.
  
  Section::::Operators.
   BULLET::::- taking precautions when track maintenance work is performed in hot weather, and making sure ballast is sufficiently consolidated before full-speed operation is resumed.
   BULLET::::- "Depression bars" or "treadles" to detect vehicles;
  
  BULLET::::- "Stainless steel strips" (often zig zag in shape) welded on the railheads;
  
  BULLET::::- "High voltage impulse" track circuits;
  
  BULLET::::- "Track Circuit Assistor (TCA)" - a train mounted system that breaks down the resistance of the rust layer;
  
  BULLET::::- "Axle counters" over the affected section; and/or
  
  BULLET::::- "tunnel sticks" whereby a track circuit cannot pickup unless a train is detected in the next track circuit.
  
  Section::::Failure modes and prevention.:Scale.
   Section::::Operation.:Structural fence.
  
  The structural fence is a physical barrier designed to stop falling rocks from reaching the tracks. Several construction methods are used, including: steel I-beams, wooden barriers, galvanized fencing, and netting directly against the rock.
  
  Section::::Where used.
   These detectors either inject frequencies into the rail, or rely on existing track circuits, to detect rail breaks. These detectors are most commonly used by high speed networks.
  
  Section::::Sensors.:Rail Temperature Monitors.
  
  These detectors are a mesh of small detectors installed against a particular stretch of rail. The detectors are typically a mix of temperature sensors and strain gauges (so measure in degrees Celsius and kilonewtons force). They measure temperature and stress/tension on the rail, to make sure these measures do not go outside the bounds of structural integrity. 
   The term "permanent way" also refers to the track in addition to lineside structures such as fences.
  
  Section::::Structure.
  
  Section::::Structure.:Traditional track structure.
  
  Notwithstanding modern technical developments, the overwhelmingly dominant track form worldwide consists of flat-bottom steel rails supported on timber or pre-stressed concrete sleepers, which are themselves laid on crushed stone ballast.
   Most rapid transit systems use conventional standard gauge railway track. Since tracks in subway tunnels are not exposed to rain, snow, or other forms of precipitation, they are often fixed directly to the floor rather than resting on ballast, such as normal railway tracks (Amsterdam being an exception). Sections of the light rail system in San Diego, California operate on former railroad rights of way that were acquired by the local transit agency.
   As an alternative to the insulated joint, audio frequency track circuits can be employed using a tuned loop formed in approximately of the rail as part of the blocking circuit. Some insulated joints are unavoidable within turnouts.
  
  Another alternative is the axle counter, which can reduce the number of track circuits and thus the number of insulated rail joints required.
  
  Section::::Joining rails.:Continuous welded rail.
  
raw_presuppositions: Train should not stay grounded to the floor.
raw_corrections: Weight of train keeps the train on the track.