If you want to travel from Sungai Petani to Penang Island (Georgetown), you have a choice of taking the KTM Komuter train (commuter), or a bus to Butterworth Bus Terminal (Penang Sentral).
The quickest and cheapest way to travel from Sg. Petani to Penang is to take one of the regular KTM Komuter train services that depart from Sungai Petani Railway Station for Butterworth Raiilway Station (please note: the ETS train no longer operates on this route).
The train station in Sungai Petani is located just off the main road that travels through town (just head towards the clock tower and take the pedestrian bridge over the railway tracks to get to the station entrance).
The duration of the journey by train is just 34 minutes, with services departing every hour during the busy periods of the day, and every two hours at other times.
Once you arrive at Butterworth Station, you have a five minute walk to the ferry terminal, where boats depart for Georgetown on Penang Island.
Tickets on the commuter train cost just 3.60 RM.
No advance booking is available for commuter services as these services are like tube / subway trains. You just buy your ticket and get on the next train and hope you can get a seat, otherwise you will have to stand.
If you don't want to take the train, the main long distance bus station in Sungai Petani is five minutes walk south of the train station (along the same road).
From here you can take coaches / buses to Butterworth Bus Station, then the ferry to Penang Island.
Cityliner operate a regular bus from Sungai Petani to Butterworth taking 40 minutes to travel the 45 km distance. Cityliner buses depart every 40 minutes or so from 06:00 until 20:40.
You can also take a Rapid Penang Bus from Sungai Petani to Butterworth, which depart every 45 minutes or so (from 06:00 until 21:00) taking around an hour to complete the journey (stops a lot more to pick up passengers along the route).
Another bus operator that operates on this route is Transnasional.
The cheapest bus fare is 4.00 RM (for the Rapid Penang bus).