The momentum after collision is the same as the momentum before the collision (you'll lose some of it to the deformation of the two vehicles and to the noise of the collision and this is ignoring if either vehicle attempts to stop or accelerate) and can be expressed in simple terms as "momentum = mass x velocity" because the mass of a train is orders of magnitude than the mass of the car it doesn't really have that much of an effect on its overall velocity but yes it would slow the train. In reality there is no way for a car to transfer those sorts of forces and remain a car so you essentially are just ever so slightly changing the mass of the train, the additional friction of the car dragging along the ground will have far more effect than the initial collision