2. Figure Q2a shows a lightweight multiple-unit passenger train comprising 3 identical carriages. Figure Q2b shows an equivalent 3DOF lumped-mass model for the train, showing masses m, m2, and m3. The masses are linked by two pairs of identical linear springs in parallel, each of stiffness k; and identical linear dampers, each of damping coefficient c. External forces f(t), and f3 (t) are shown to represent the action of braking applied to the front and rear carriages i.e. to m, and m3. i) 0 0 0 0 Figure Q2a free-body diagrams AAA f(t) m1 m2 W m3 f3(t) Figure Q2b