Who produces the trains operated by the MTR?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/operator> <http://dbpedia.org/resource/MTR> . ?x <http://dbpedia.org/ontology/manufacturer> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Train>}
MTR