The second quantity is known as the correlation function. The delta is the kronecker delta which tells us that if the indices "i" and "j" are different, then the result is 0, but if "i" = "j" then the kronecker delta is 1, so the correlation function returns a value of "b"2. This makes sense, because if "i" = "j" then we are considering the same step. Rather trivially then it can be shown that the average displacement of the train on the x-axis is 0;