• main mesh contains all triangles of this mesh and also all that can be accessed via submeshes.
• submeshes contains tris that can be accessed also via main mesh
• submeshes are always exists in any mesh, at least one
• sum of all submeshes tris is equal to main mesh tris
• submeshes are used to apply different materials to tris

so finally, when you got from raycast hit to triangle index 50, you should search it in main mesh (it contains all tris including triangle 50), then get vertices for this triangle, and find triangle with those vertices in submeshes

bonus: MeshAnaliser.cs

using UnityEngine;
using System.Collections;
public class MeshAnaliser : MonoBehaviour
{
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            RaycastHit hit;
            if (Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit))
            {
                Mesh m = GetMesh(gameObject);
                if (m)
                {
                    int[] hittedTriangle = new int[] 
                    {
                        m.triangles[hit.triangleIndex * 3], 
                        m.triangles[hit.triangleIndex * 3 + 1], 
                        m.triangles[hit.triangleIndex * 3 + 2] 
                    };
                    for (int i = 0; i < m.subMeshCount; i++)
                    {
                        int[] subMeshTris = m.GetTriangles(i);
                        for (int j = 0; j < subMeshTris.Length; j += 3)
                        {
                            if (subMeshTris[j] == hittedTriangle[0] &&
                                subMeshTris[j + 1] == hittedTriangle[1] &&
                                subMeshTris[j + 2] == hittedTriangle[2])
                            {
                                Debug.Log(string.Format("triangle index:{0} submesh index:{1} submesh triangle index:{2}", hit.triangleIndex, i, j / 3));
                            }
                        }
                    }
                }
            }
        }
    }
    static Mesh GetMesh(GameObject go)
    {
        if (go)
        {
            MeshFilter mf = go.GetComponent<MeshFilter>();
            if (mf)
            {
                Mesh m = mf.sharedMesh;
                if (!m) { m = mf.mesh; }
                if (m)
                {
                    return m;
                }
            }
        }
        return (Mesh)null;
    }
}