How to change velocity of object which move by Rigidbody.

I made a roller coaster moving along the rails. This roller coaster is driven by a rigid body (kinetic energy, position energy). I want this roller coaster to slow down or speed up when I hit a certain key. How can I do?

(like gas pedal or brake)

private Rigidbody rb;

void Start(){
   rb = GetComponent<Rigidbody>();

void Update(){
    if(Input.GetKeyDown(KeyCode.D){ // we speed up
        rb.velocity *= 1.1f;
   }else if(Input.GetKeyDown(KeyCode.A) // we slow down
        rb.velocity *= 0.9f;

If you want to go back to normal speed just do rb.velocity *= 1. Im also working on roller coaster and works fine.