A cheap way to travel from Penang to Thailand by train is to take the regular commuter service which departs every hour or two from Butterworth Railway Station on the mainland.
If you are staying on Penang Island, you will first have to take the Penang to Butterworth ferry over to the mainland, where it is just a short walk to get to the railway station.
Commuter Timetable for 2019 (All Trains operate daily):
Morning / Afternoon Timetable
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Afternoon / Evening Timetable
You can also use these trains to connect to the other Komuter Utara Line (northern commuter line) that operate from Bukit Mertajam to Perak state (terminating at Padang Rengas).
Komuter trains are not like the more comfortable Electric Train Services and seating is arranged along the sides of the carriages like a subway train.
For those unlucky enough to not get a seat, you will have to stand in the centre of the aisle.
If you get on at Butterworth, chances are you will be fine and get at seat most of the time, but be warned, this Komuter route is very popular and during busy holidays periods, trains can get packed, especially in the middle section of the route around Alor Setar.
Another thing you should be aware of: there are no toilets on board.
The KTM Komuter trains are the cheapest way to travel in this area of the country with prices much lower than the same journey by bus.
Tickets for these commuter trains are only sold on the day of travel.
Tickets are available from the ticket counter at Butterworth Station (not available from the KTM office in Georgetown), so allow yourselves some extra time to buy tickets, as you may (at busy times of the day) have to queue at the counter.