In the Industry, the Modal Analysis or Natural Frequency Analysis or Eigenvalue Solution of the Structures is most widely accepted to avoid the resonance of the Structure (Component / Assembly). Finding the Mode Shapes of the Structure is also a prime importance. When the Structure is excited with the Frequencies which are very close to the Natural Frequencies may lead to catastrophic failure of the structure. Apart from this, many structures are designed to work under Stress. When performing the Modal Analysis on such Structures, the Pre-Stressed Effect must be considered, because the Stress State Changes the Natural Frequency of the Structure. The Response of the Structure without the Pre-Stress Effect may be very different from the Response of the Structure with the Pre-Stress Effect. The Differential Stiffness [adding a Stress Stiffness Matrix – may be due to Linear or Non-Linear] is considered while finding out the Eigenvalue Solution. Both Free-Free and Pre-Stressed Modal Analysis results in Natural Frequencies and Mode Shapes but under different conditions. If Pre-Stressed Modal results differ appreciably from design intent, one must understand which parameters are causing this change in modal results and must be optimized accordingly. The Elastic Rail Clip (ERC – MARK-III – T3701) is studied for the Pre-Stressed Effect and its effect on the Mode Shapes and the Natural Frequencies. The Frequencies and Mode Shapes of Pre-Stressed Modal Analysis differ than the Free-Free Modal Analysis due to the effect of Differential Stiffness comes into the picture when the ERC in situ.