Maximum joint expansion gap


A rail joint is a track component that connects two pieces of rail and it provides room for rail expansion and contraction due to the changes in temperature.

The mechanical rail joint is designed to allow a gap variation and, as such, has two well defined limits of this gap.


The minimum gap of a mechanical rail joint is  null and that is usually achieved without exerting  shear force on any of the joint bolts.

The maximum expansion gap of a mechanical rail joint is dependant on the position of the joint holes and on the diameter of the holes and bolts of the joint (Radu, 1999). This maximum gap is reached when the rails contract due to temperature decrease and are starting to exert shear forces on the joint bolts.

The maximum expansion gap Gmax can be computed as:

Gmax = Bf + Df + Dr – 2Db – 2Br


  • Bf – the distance between the centres of the middle holes of the fishplate;
  • Df – the fishplate hole diameter;
  • Br – the distance from the end of rail to the first rail hole centre;
  • Dr – the rail joint hole diameter;
  • Db – the joint bolt diameter.


For a mechanical joint there will always be this relation between the joint holes and bolt diameters : Db < Df < Dr.

The maximum gap Gmax is designed for the longest rail used on jointed track to avoid the shearing of the joint bolts when the rail contracts; it is an important parameters in the analysis of the jointed track behaviour due to rail temperature variations.

Considering this maximum gap formula and the standards defining the dimensions of the rail joint components, in the table below are computed the maximum gaps for a few mechanical joints used in UK:

Later edit (15.08.2016): The table shown previously was quoting a wrong bold dimension and not the nominal value shown in the BSI BS 64 standard. Now this is corrected. 


The long rail definition is explained in another blog post here: Long rail.

Below is a simplified schematic animation of a joint expansion cycle, for a continuous rail temperature variation between tmin and tmax:

More details about this expansion cycle will be presented in a future article published in the Journal of the Permanent Way Institution (PWI).


Cope, Geoffrey H. (1993). British railway track: design, construction and maintenance. Permanent Way Institution.

Radu, Constantin (1999). Cai Ferate – Suprastructura Caii (Railway – Track Superstructure) – Course notes, Faculty of Railways, Roads and Bridges – Technical University of Civil Engineering Bucharest

BSI BS 11:1985. Specifications for railway rails. British Standards Institution.

BSI BS 47-1:1991. Fishplates for Railway Rails – Part 1: Specification for Rolled Steel Fishplates. British Standards Institution.

BSI BS 64:1992. Specification for Normal and High Strength Steel Bolts and Nuts for Railway Rail Fishplates. British Standards Institution.

2 thoughts on “Maximum joint expansion gap

  1. Hi Onur.
    Thank you for your comment.
    Usually the insulated rail joints are designed to not allow any relative movements between rails.
    Even though often the rail holes sometimes might be similar to the ones of an usual mechanical joint, the difference between the rail hole and the bold diameter is compensated by the use of an insulator ring. This doesn’t allow the rail to get in contact with the bolt and create an electrical connection, through the bolts and fishplates, to the other rail.
    Another elements that’s usually different at the IRJ relative to the mechanical joint is the type of bolts used for the joint.
    The (modern) mechanical joint has its bolts designed to give a certain longitudinal resistance to rail movement; this joint resistance is around 5000 daN for a four bolted mechanical joint.
    The IRJ is often blocked by the use of high tensile bolts, which are giving a much higher joint longitudinal resistance – otherwise the insulator rings I mentioned above, and the insulator placed between rails will be crushed by the rail thermal breathing; the joint will be then not fit for purpose, allowing electrical continuity through its section.

    Long story short – usually the insulated rail joints are blocked and in normal condition there should be no gap variance due to rail thermal breathing. The jointed rail should behave as a continuous long rail.

    I hope this answers your question.


  2. Thanks for this useful info.
    I would like to know also that is there any technical details for insulated rail joints in terms of allowable I will be glad to know more details about IRJ….


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