Moving object down or up the spline using gravity

[velikizlivuk]

I ended up using this:

Code:

public class Cart_MotorController : SplineController
    {
        [Section("Motor")]
        public float MaxSpeed = 30;
        public float RailSlopeFactor = 10;
        [SerializeField] private float slopeInducedSpeed = 0;
        [SerializeField] private float slopeInclination = 0;
        private float axis = 0;
        [SerializeField] private bool isPlayerInCart = false;


        protected override void FixedUpdate()
        {
            axis = Input.GetAxis("Vertical");
            slopeInclination = GetTangent(RelativePosition).y; // minus forward slope, plus backward slope

            if (slopeInclination < -0.05f || slopeInclination > 0.05f)
            {
                slopeInducedSpeed = Mathf.Abs(slopeInclination) * RailSlopeFactor;               
            }

            if (isPlayerInCart)
            {
                MovementDirection = MovementDirectionMethods.FromInt((int)Mathf.Sign(axis));
            }
            else
            {
                axis = 0;
                MovementDirection = MovementDirectionMethods.FromInt((int)Mathf.Sign(-slopeInclination));
            }

            Speed = Mathf.Lerp (Speed, (Mathf.Abs(axis) * MaxSpeed)+slopeInducedSpeed, Time.fixedDeltaTime*1.5f);

            if (Speed<0.01f)
            {
                Speed = 0.0f;
            }

            base.FixedUpdate();

        }

        private void OnTriggerEnter(Collider other)
        {
            if (other.gameObject.CompareTag("Player"))
            {
                isPlayerInCart = true;
            }
        }
        private void OnTriggerExit(Collider other)
        {
            if (other.gameObject.CompareTag("Player"))
            {
                isPlayerInCart = false;
            }   
        }

    }

Needs more polishing but it works as inteded.
I'm using rigidbody on my rail cart and player so using FixedUpdate as General/Update in and Kimenatic Rigidbody as General/Target component in Motor component.