There are several ways to set initial conditions. These positions were chosen such that the system is a rest. In this case the system is not at rest. Use of model accelerate. Use of damper models.
Oscillator demonstrates the use of initial conditions.
Information A spring - mass system is a mechanical oscillator. If no damping is included and the system is excited at resonance frequency infinite amplitudes will result. If damping is added the amplitudes are bounded. Examples .Sensors Sensors for translational systems. Plot PositionSensor1.s, PositionSensor2.s and SlidingMass1.s to see the difference.