The main principle of the FASTSIM algorithm is the use of the relation between the tangential stresses and the displacement based on the simplified elasticity theory instead of the use of the elasticity theory developed by Cerruti.

you can find the tangential forces acting at each contact point by knowing the following parameters:

*  relative slip in the lateral direction y.

*  longitudinal creepage.

*  lateral creepage.

* spin creepage.

* deformational displacement in the longitudinal direction.

* deformational displacement in the lateral direction.

see the following references 

1- A.Rovira_2011(( Experimental and numerical modelling of wheel–rail contact and wear))

2- A.Rovira_2012 (( Application of Fastsim with variable coefficient of friction using twin disc experimental measurements))

May be this material can help you:

1. http://www.um.rssi.ru/download/50/eng/08_um_loco.pdf

 2. https://www.researchgate.net/profile/Enrico_Meli/publication/241686357_Development_of_a_wear_model_for_the_prediction_of_wheel_and_rail_profile_evolution_in_railway_systems/links/0deec51c9a9bac79c7000000.pdf

Article Development of a wear model for the prediction of wheel and ...

Actually, you can get the stress of each cell  at the contact patch(as shown in the figure you attached) from FASTSIM, the resultant force can be obtained by integration over the contact patch. It is not difficult to implement the algorithm in matlab, please refer to 'a fast algorithm for the simplified theory of rolling contact' by Kalker. A fortran code was provided as well in the paper.

thank you all so much for your help...i really appreciate. i have solved my problem now.

cheers