The aerodynamic forces acting on representative cars of high-speed trains travelling in a tunnel have been studied theoretically, as well as the translational and rotational motions of these cars. The aerodynamic forces are simplified into one-dimensional sinusoidal ones. Each of the cars, coupled to others and supported by springs and dampers, has two degrees of freedom for translational and rotational motions. The results show that (a) the calculated aerodynamically-induced lateral force and yawing moment agree well with observations from running experiments; (b) the steady-state mode shapes agree well with those obtained previously from running experiments; and (c) the wavelength of the travelling aerodynamic force controls the phase differences among the cars of the train.