There is need for a more detailed explanation of the question for example

1Force vertical or horizontal.

2. accelerometer location.

suggest a simple sketch be given to get meaningful answers.

Cheers

it is dynamic force analysis, as long as the train is moving, hence, you have to add Time element to your equation

So how do I get FRF of a bridge on which tain goes. Do i have to have a force sensor installed on the bridge or i can get the force function from the acceleration sensors I have installed on the bridge??