You can leave after you have finished registering for classes and your advisor has signed you off. The earlier your registration time, the earlier you can leave. When my son went to orientation 2 years ago, his registration time was 2:30 and was finished by 3. If you have a train to catch and you signed up for the shuttle, you should be fine with your registration time. When you sign in for orientation, just double check to make sure you have plenty of time to catch the train.
Make sure you have a list of classes ready for your registration time along with some alternates in case some of the classes are closed. If you are not ready for registration, it will take you much longer.