Each domino is a rigid slab pivoting about its front-bottom edge:
theta'' = [ g (h/2 sin theta - t/2 cos theta) - fr theta' + contacts ] / ((h^2 + t^2)/3)
The tip of domino i touches the back face of i+1 when h sin(theta_i) exceeds the gap + h cos(theta_i) tan(theta_{i+1}). A stiff damped spring transmits the push and torque. If the gap is too wide or friction too high the chain stalls; otherwise a wave of constant speed travels down the line.