How do I move my train with a slider, and not W & S

You know how mobile train simulators use a slider to move the train? I would like to emulate it. I found this script on one of my VERY old simulators. It rotates sphere collides, which in turn moves the train. I like the way the train gradually accelerates, and decelerates realistically, and brakes realistically. However, as I want to adapt this game for mobile, I need to make it so the train moves with a UI slider.

 using UnityEngine;
 using System.Collections;
 
 public class Drive_Wheel : MonoBehaviour {
 
     public bool Drive_Flag = true ;
     public float Max_Torque = 800.0f ;
     public float Accelerate_Rate = 200.0f ;
     public float Brake_Drag = 5.0f ;
     public float Idle_Drag = 0.1f ;
 
     Rigidbody This_RigidBody ;
     float Target_Torque ;
     float Current_Torque ;
 
     void Start () {
         This_RigidBody = this.GetComponent<Rigidbody>() ;
         This_RigidBody.maxAngularVelocity = 50.0f ;
         This_RigidBody.angularDrag = Idle_Drag ;
         // Layer settings.
         this.gameObject.layer = 8 ;
         Physics.IgnoreLayerCollision ( 0 , 8 , true ) ;
         Physics.IgnoreLayerCollision ( 8 , 8 , false ) ;
     }
 
     void Update () {
         // Accelerate
         if ( Drive_Flag ) {
             if ( Input.GetAxis ( "Vertical" ) > 0.0f ) {
                 Target_Torque = Mathf.Lerp ( 0.0f , Max_Torque , Input.GetAxis ( "Vertical" ) ) ;
             }
         }
         // Brake
         if ( Input.GetAxis ( "Vertical" ) < 0.0f ) {
             Target_Torque = 0.0f ;
             This_RigidBody.angularDrag = -Input.GetAxis ( "Vertical" ) * Brake_Drag ;
         }
         // Idle
         if ( Input.GetAxis ( "Vertical" ) == 0.0f ) {
             Target_Torque = 0.0f ;
             This_RigidBody.angularDrag = Idle_Drag ;
         }
         //
         if ( Current_Torque < Target_Torque ) {
             Current_Torque = Mathf.MoveTowards ( Current_Torque , Target_Torque , Accelerate_Rate * Time.deltaTime ) ;
         } else if ( Current_Torque > Target_Torque ) {
             Current_Torque = Target_Torque ;
         }
     }
 
     void FixedUpdate () {
         GetComponent<Rigidbody>().AddRelativeTorque ( Current_Torque , 0 , 0 ) ;
     }
 
 }

Thanks guys!