Typical railroad ties manufactured from wood require frequent replacement due to exposure to the environment, including weather, insects and micro-organisms, all of which can shorten the life of a wooden tie.  Wooden ties may also be chemically treated to lengthen their life, but such treatment may raise environmental concerns, and adds to the cost of manufacturing the tie.  It is known to manufacture ties from a plastic or composite material, which alleviates the problems associated with wooden ties, but which also causes problems not associated with wooden ties. 
Ties made of wood tend to settle into the ballast, typically rocks, over a period of time and repeated loadings, and, because the properties of wood orthogonal to the long axis of the tree and tie are much weaker than the properties along the axis, the ties become naturally dimpled on the bottom as they settle into the ballast.  This dimpling, and the related mechanical interaction between the wooden ties and the ballast tend help to keep the tie anchored in place. 
In the U.S., a typical railroad tie is rectangular in shape, having a cross section 7 inches in height by 9 inches in width.  Railroad ties manufactured from plastics or composites are typically the same size and shape as ties made of wood, and must meet the same structural specification as wooden ties.  Specifically, the tie must not allow an increase in the gauge of the tracks by more than 0.125 inches under a lateral load of 24,000 lbs.  and a static vertical load of 39,000 lbs.  In addition, the tie must be able to withstand a dynamic vertical load of 140,000 lbs. 
The mechanical properties of plastic and composite ties may prevent these ties from becoming dimpled and indented with ballast over time as occurs with wooden ties.  To overcome this, ties manufactured from plastics or composites sometimes have a pattern embossed or imprinted on the bottoms and sides to allow increased mechanical interaction with the ballast, such as to emulate the effect which occurs naturally with wooden ties. 
Unfortunately, these plastic and composite ties have demonstrated that a tendency to become “center bound”, which makes them prone to cracking in the middle of the tie.  A center bound tie is one that is supported underneath with a higher mound of ballast in the center of the tie than exists at the ends of the tie or under the rails.  This causes the ties to flex along the longitudinal axis and, to a somewhat lesser extent, along the axis orthogonal to the longitudinal axis, every time that the tie is loaded by a train moving over the track.  This eventually causes the tie to crack, and as a result, the tie is unable to hold gauge with the rails.  Therefore, it would be advantageous to have a tie composed of a plastic or composite material which is shaped to alleviate the center bounding problem.