By Trains to other parts of Malaysia and Thailand

One of Penang’s main rail link is the Butterworth KTM Station, on the mainland, located next to Penang Sentral and accessible by a free three-minute shuttle bus from the ferry terminal. (If you fly into Penang Island, you have to take a taxi, Grab or bus to the ferry terminal.).
Penang Sentral has a reservation ticketing system and self-service ticket vending machines. There are also many shops and F&B outlets within.
How to board: After buying your ticket and going through security, head to the lower ground floor, where the bus bays are and where you can reach the station foot, less than a minute away.
[Important Note: The other main train terminal in Penang is at Bukit Mertajam. Most of the North-South rail link of the ETS services serving KL and Padang Besar stops at the town of Bukit Mertajam, near Butterworth.]

However, you have three travel options by train if you start from the Butterworth station.

ETS from Butterworth to KL
The first option is taking the ETS(Electric Train System). The KTM ETS is an inter-city rail service operated by Keretapi Tanah Melayu Berhad utilizing electric multiple units. The ETS lines, services to and from KL and Padang Besar, reaching KL Sentral in four hours and four-and-a-half hours, respectively. ETS Butterworth to KL has Platinum and Gold services,Tickets cost RM79 (Platinum) and RM59 (Gold), the difference being that Gold services make more stops at more stations along the way. There are only 4 train services per day to and from KL. All ETS from Butterworth to KL stop at Ipoh

KTM Komuter from Butterworth to Perak
The second option of trains from Penang is the southbound KTM Komuter Train from Penang to Taiping and Kuala Kangsar. KTM Komuter trains are for locals to travel easily and cheaply to neighbouring towns all the way to Perak. They cost a fraction of the ETS (about RM10 to Taiping) and are not that much slower either.
The trains run only twice a day at 7am and 7pm, but you can take any of the hourly KTM Komuter services heading towards Padang Besar and alight at Bukit Mertajam, where the KTM trains to Taiping/Padang Rengas run roughly every two hours, from 5.42am to 9.42pm.

Malaysia to Thailand
The third option from Butterworth is the northbound KTM Komuter Train to Padang Besar, at the Malaysia-Thailand border. Trains leaves hourly from 5.25am to 9.25pm (except at 11am, 1pm and 8pm).
It reaches Padang Besar in one hour and 40 minutes, and only costs RM11.40. This train is by far the cheapest, easiest, fastest and more scenic way to reach the Malaysia-Thailand border.
Possible itinerary to Bangkok:
1) Take the KTM service to Padang Besar which leaves Butterworth at 2.25pm. You will alight at Padang Besar station (Malaysian side) at 4.16pm. Then head upstairs to buy a through ticket on the international train to Bangkok that leaves Padang Besar at 5.30pm.
2) Hat Yai to Bankok by train: You can choose may train 'classes' at Hat Yai station. To get to Hat Yai from malaysia, you can catch a minivan from the border for 50 baht, or wait for one of the two daily train shuttle connections between Padang Besar and Hat Yai, which leave at 9.55am and 3.40pm.

Additional information