Platform.cs

using UnityEngine;
using System.Collections;

public class Platform : MonoBehaviour {

    public float heightToRaise = 5;
    public float time = 5;
    private bool up = false;

    void Start () {
        StartCoroutine("Oscillate");
    }

    void Update() {
        transform.Translate(new Vector3(0,(up ? 1 : -1)*heightToRaise/time*Time.deltaTime,0));
    }

    IEnumerator Oscillate() {
        while(true)
        {
            up = !up;
            yield return new WaitForSeconds(time);
        }
    }
}