Taken 2nd class sleeper south twice, north once. Here are the notes I kept:
Best seats are 13-24, the middle of the car. Car ends are noisey and the toilets are at the seat number 1 end of the car.
My ticket for the return trip said the train left Butterworth at 13:15. The station's signboard said 13:45. Other people's tickets said 14:20, which was when it departed.
Don't expect any announcement of what station you're pulling into
Opportunities to buy food and water at station stops was limited. Next time I'll bring all the water I expect to need.
Walk from end of train to the ferry is about 400 yards. Exit the train station, look to left for a covered elevated walkway. There are no signs, probably because the taxi drivers tore them down.
No shortage of buses or taxis at Ferry terminal in Penang.
Lower berth is much wider than the upper.
Trash lay where it fell during the Malaysian portion. There is a trash can in the car, but we started north from Butterworth with it full to overflowing. Next time I'm bringing some plastic bags for my own use.
For a Malaysian portion of the trip one's second class air con sleeper ticket becomes second class air con. That means what was once your seat becomes only half your seat. Kids apparently don't need seperate tickets, so you can end up seated across from a couple with two kids.
The train workers want to get the berths made up early because it's easier on them. During meal time a worker will offer to set up a table between the two seats. Keep the table after the meal, then keep something on it like food, a card game, laptop etc. and the guy who wants to make up the berths at 9pm can be talked out of it. Eventually.
Detailed info on all things train: http://www.seat61.com/Thailand.htm