Redirect to action with JsonResult

If you use AJAX to request a page, it's cant redirect in browser. You should response a status code, and then use javascript to redirect in front, like this

[HttpPost]
public JsonResult GetUserTraj()
{
    if (Session["UserName"] != null)
    {
        var userTrajList =
            DBManager.Instance.GetUserTraj(Session["UserName"].ToString());
        return Json(userTrajList);
    }
    else
    {
        //RedirectToAction("Login", "Login");
        return Json(new {code=1});
    }
}

You need write this condition Inside of your Ajax success call to reload login screen,

if(result.code ===1){
    window.location = 'yourloginpage.html'
}

You can't redirect user to a new page using ajax. For this you have to send some flag at client side and then need to use that flag to identify that session has been expired. Following code will help you:

[HttpPost]
public JsonResult GetUserTraj()
{
    if (Session["UserName"] != null)
    {
        var userTrajList = DBManager.Instance.GetUserTraj(Session["UserName"].ToString());
        return Json(new { Success = true, Data = userTrajList});
    }
    else
    {
        return Json(new { Success = false, Message = "Session Expired"});
    }
}

jQuery

$.ajax({
  url: "any url",
  dataType: '',
  contentType: "------",
  success: function(response){
    if(response.Success){
     // do stuff
    }else{
    window.location.href = "/YourLoginURL.aspx"
    }
  }
});