On the way to Bangkok by train from Kuala Lumpur, while waiting the train to Bangkok from Butterworth, you can to go to Penang Island by ferry. The Butterworth Jetty is 10-15 minutes walk from the train station. The ferry ride will take 10-15 minutes to reach Georgetown Jetty in Penang Island. There is a bus station next to jetty where you can board a bus to Penang Hill (bus number 204).
Here are some photos: