Simplify the following text for a Grade 12 native English-speaking person.
Original: After World War II Szell often conducted other famous orchestras , including the Concertgebouw Orchestra , the London Symphony Orchestra , the Vienna Philharmonic , the New York Philharmonic and at the Salzburg Festival .
Simplified: After World War II Szell became closely associated with the Concertgebouw Orchestra of Amsterdam , where he was a frequent guest conductor and made a number of recordings . He also regularly appeared with the London Symphony Orchestra , the Vienna Philharmonic , and at the Salzburg Festival .