Hi Enna...caught the overnight train last year from Arau station I think it was...booked about 3 days in advance and didn't have any problems.
In July, took the day train from Butterworth to KL. You can catch the ferry from Georgetown across to Butterworth. When everyone gets off the Butterwoth side they all head to the left to the waiting buses...if you go right along the undercover walkway for a few hundred metres you'll end up at the train station aand can book tickets there.
Its recommended to book before you arrive, as seats can be limited, I also advise a 1st class seat. Although Malaysian trains are a step below their Euro counterparts they are clean and comfortable but not what you may call "express". Bookings can be made anytime 2 months prior on their website below.
The following is also very good reading and should tell you all you need to know.
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