It is necessary to have equations of motion for the numerical simulation of a dynamic system such as mechanical ones. One of the methods to derive equations of motion is to use the Lagrange's equation. However it is not easy to apply the Lagrange's equation to the system that includes linkage mechanism. And, the Lagrange's method usually needs to do the derivation of equations of motion over again from the beginning even when the object model is modified partly. This paper proposes the employment of the Bond-Graph modeling to derive the equations of motion for mechanical systems with linkage mechanism. A forced steering railway vehicle is considered as an example of the application. In the example, steering linkage mechanism is added to conventional vehicle model whose equations of motion are already known. Using the, Bond-Graph modeling only for the added linkage part, the modification of the related equations of motion is achieved. Here, two kinds of linkage mechanism are examined, and the validity of the modified equations is demonstrated in the numerical simulation. Then it is confirmed that the Bond-Graph modeling is useful to derive the equations of motion of a rail vehicle added with some linkage mechanism.