Here is your working Ajax. I tested it and it gives information correctly about pnr enquiry.
$.ajax({
type: "GET",
crossDomain:true,
url: "http://pnrbuddy.com/api/check_pnr/pnr/2156641988/format/json/pbapikey/6b17f33e25e2d8197462d1c6bcb0b130/pbapisign/bd0aea241e88c8a22692eba02887ad97a220f827",
success: function (data) {
alert(data.pnr); // result - 2156641988
alert(data.train_num); // result - 14208
alert(data.train_name); // result - PADMAVAT EXPRES
}
});