A cheap and convenient way to travel to and from Sungai Petani is to take the regular KTM Komuter trains (commuter train).
In this part of Malaysia, the Northern Commuter Line is called Komuter Utara and there are two different routes you can use to get around the states of Kedah, Perlis, Penang and Perak.
The two Komuter Utara Routes are:
1) Butterworth - Padang Besar route (to the border of Malaysia with Thailand).
2) Butterworth / Bukit Mertajam to Padang Rengas (Perak state).
To get from one route to the other, change trains at Bukit Mertajam.
Ticket Price List:
Sungai Petani to Gurun KTM fare = RM 2.30
Sungai Petani to Kobah KTM fare = RM 4.20
Sungai Petani to Alor Setar KTM fare = RM 5.00
Sungai Petani to Anak Bukit KTM fare = RM 5.50
Sungai Petani to Kodiang KTM fare = RM 6.90
Sungai Petani to Arau KTM fare = RM 7.40
Sungai Petani to Bukit Ketri KTM fare = RM 8.10
Sungai Petani to Padang Besar KTM fare = RM 9.30
Sungai Petani to Tasek Gelugor KTM fare = RM 2.00
Sungai Petani to Bukit Mertajam KTM fare = RM 2.40
Sungai Petani to Bukit Tengah KTM fare = RM 2.80
Sungai Petani to Butterworth KTM fare = RM 3.60
Ticket Price List:
Sungai Petani to Simpang Ampat KTM fare = RM 3.30
Sungai Petani to Nibong Tebal KTM fare = RM 4.50
Sungai Petani to Parit Buntar KTM fare = RM 4.80
Sungai Petani to Bagan Serai KTM fare = RM 5.40
Sungai Petani to Kamunting KTM fare = RM 7.80
Sungai Petani to Taiping KTM fare = RM 8.10
Sungai Petani to Padang Rengas KTM fare = RM 9.40
If you want to travel by KTM Komuter train to/from Sg. Petani you just have to turn up at the station and buy a ticket for the next train.
There are no advance ticket reservations available for commuter trains in Malaysia and as these services are like subway / tube trains, seats cannot be reserved.
So, you may have to stand for a while on the busy sections of the line.