state space model of the vehicle system

The inputs to the model is the track input which is a step signal.Now I don't get any error but since the input matrix is zero ,I'm not able to get the expected response(which is a difference of the track input and the wheelset position) rather it shows a line at 0. The matrix size is given as,

A= 14x14
B=zeros(size(A,1),2)
C=eye(size(A))
D=zeros(size(A,1),2)

I don't understand how the input is taken by the state space model and how to define the size of the input matrix.

If you need any other information please let me know.