If you are looking to travel from Singapore to Langkawi by train, there are no direct KTM services to the closest ferry terminals for Langkawi Island, of either Kuala Kedah (near Alor Setar) or Kuala Perlis (near Arau), so you will have to make a couple of stops along the way.
The first step of the journey is to get from Singapore to JB Sentral Railway Station in Johor Bahru, Malaysia.
JB Sentral is close to the main border crossing between Singapore and Malaysia.
You can choose to get there by bus or by Shuttle Train from Woodlands Railway Station in Singapore.
From JB Sentral you will then have to catch a Shuttle or Intercity Train to Gemas (where the dual electrified railway tracks start).
From Gemas you will then have to change for one of the new high-speed Electric Train Services (ETS) heading to Padang Besar.
You will now have to decide at which station you want to disembark for the ferry over to Langkawi (Alor Setar or Arau).
The first station you come to is Alor Setar : from here you have to travel to the port of Kuala Kedah.
From the station in Alor Setar, you can either get a taxi or take a walk into the town centre and catch a very cheap local bus.
The next option is to stay on the train a little longer and disembark at Arau.
From Arau, you will have to travel to the port of Kuala Perlis.
You will have to take a taxi from the station to the jetty, as buses are very infrequent and will only get you to Kangar.
This makes it a more expensive option, but you will save a few RM on the ferry fare, as boats from Kuala Perlis Jetty to Langkawi are around half an hour shorter than from K. Kedah (so are a little cheaper).
Another option is to travel from Singapore to Langkawi, via Penang.
You will again first have to travel to Gemas by Shuttle Train, then from there connect to a Gemas to Butterworth ETS Train.
Butterworth Railway Station is located just a short ferry ride over to the island of Penang and the old heritage city of George Town.
If you want to travel from Singapore to Langkawi this way you have a few options open to you once you arrive at Butterworth railway station, but you will have to spend at least one night in Penang.
You can continue your journey to Langkawi by either of the following routes :
You can catch a high-speed ferry service from George Town to Langkawi.
There are two departures every morning and one in the afternoon (seasonal).
Or take a cheaper option and catch a bus or train from Butterworth to Alor Setar and then another local bus from there to the ferry terminal at Kuala Kedah.
Or take a bus from Butterworth to Kuala Perlis.
Boats from these ports are much cheaper than from Penang as the distance over to Langkawi is much shorter.
If you want a more direct way to travel from Singapore to Langkawi (apart from flying) you can get a bus from Singapore to Alor Setar or from Singapore to Kuala Perlis port.