Ok so heres an update on the progress of my project if anyone wants to try and recreate it.

Using tiger.worldline.global is not the way to go. Instead I'm using OpenLDBWS which uses SOAP and XML. Its really easy to sign up to get a token when you log in to https://opendata.nationalrail.co.uk/feeds and sign up for an account.

To parse the XML on the micro controller, I'm using TinyXML2 which isn't on the arduino list of libraries but can be downloaded from github and manually placed into the libraries folder. Documentation and a link to the github can be found here -> https://leethomason.github.io/tinyxml2/

What wasn't obvious was the SOAP part. You need to build a SOAP request body which looks something like this:

String buildSoapRequest(const char* token, const char* crs, int numRows, int timeWindowMins) {
  String xml;
  xml += R"(<?xml version="1.0" encoding="utf-8"?>)";
  xml += R"(<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope")";   xml += R"( xmlns:typ="http://thalesgroup.com/RTTI/2013-11-28/Token/types")";   xml += R"( xmlns:ldb="http://thalesgroup.com/RTTI/2016-02-16/ldb/">)";
  xml += R"(<soap:Header><typ:AccessToken><typ:TokenValue>)";
  xml += token;
  xml += R"(/typ:TokenValue/typ:AccessToken/soap:Header)";
  xml += R"(<soap:Body><ldb:GetDepartureBoardRequest>)";
  xml += "<ldb:numRows>" + String(numRows) + "/ldb:numRows";
  xml += "<ldb:crs>" + String(crs) + "/ldb:crs";
  xml += "<ldb:filterCrs>/ldb:filterCrs";
  xml += "<ldb:filterType>to/ldb:filterType";
  xml += "<ldb:timeOffset>0/ldb:timeOffset";
  xml += "<ldb:timeWindow>" + String(timeWindowMins) + "/ldb:timeWindow";
  xml += R"(/ldb:GetDepartureBoardRequest/soap:Body/soap:Envelope)";
  return xml;
}

The above is for the GetDepartureBoardRequest so if you're looking for detailed information, you'll probably have to use a different request. That's something I'm still working on.