C# Unity Script (but excluding any project namespace) for showing a file dialog in Unity for selecting a .Sch train schedule file name – source taken from various websites but linked it together for a working script example here. Acknowledging Astiolo.

using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;
using System;
using System.Runtime.InteropServices;



   [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]

    public class OpenFileName {
        public int      structSize = 0;
        public IntPtr   dlgOwner = IntPtr.Zero;
        public IntPtr   instance = IntPtr.Zero;
        public String   filter = null;
        public String   customFilter = null;
        public int      maxCustFilter = 0;
        public int      filterIndex = 0;
        public String   file = null;
        public int      maxFile = 0;
        public String   fileTitle = null;
        public int      maxFileTitle = 0;
        public String   initialDir = null;
        public String   title = null;
        public int      flags = 0;
        public short    fileOffset = 0;
        public short    fileExtension = 0;
        public String   defExt = null;
        public IntPtr   custData = IntPtr.Zero;
        public IntPtr   hook = IntPtr.Zero;
        public String   templateName = null;
        public IntPtr   reservedPtr = IntPtr.Zero;
        public int      reservedInt = 0;
        public int      flagsEx = 0;
    }
     
    public class DllTest {
        [DllImport("Comdlg32.dll", SetLastError = true, ThrowOnUnmappableChar = true, CharSet = CharSet.Auto)]
        public static extern bool GetOpenFileName([In, Out] OpenFileName ofn);
        public static bool GetOpenFileName1([In, Out] OpenFileName ofn) {
            return GetOpenFileName(ofn);
        }
    }	

public class TimerClock : MonoBehaviour
{



	void Start () 
	{

OpenFileName ofn = new OpenFileName();
            ofn.structSize = Marshal.SizeOf(ofn);
            ofn.filter = "Train Sch Files\0*.sch\0\0";
            ofn.file = new string(new char[256]);
            ofn.maxFile = ofn.file.Length;
            ofn.fileTitle = new string(new char[64]);
            ofn.maxFileTitle = ofn.fileTitle.Length;
            ofn.initialDir = UnityEngine.Application.dataPath;
            ofn.title = "Upload Train Schedule";
            ofn.defExt = "SCH";
            ofn.flags = 0x00080000 | 0x00001000 | 0x00000800 | 0x00000200 | 0x00000008;//OFN_EXPLORER|OFN_FILEMUSTEXIST|OFN_PATHMUSTEXIST| OFN_ALLOWMULTISELECT|OFN_NOCHANGEDIR
            if(DllTest.GetOpenFileName(ofn)) {
                Debug.Log( "Selected file with full path: {0}"+ofn.file );
            }


	}

	void Update ()
	{
		



	}
}