Depends on what facilities you are going to have on the train. If you are sitting up through the night, I would take a pillow of some kind and a blanket. Also hope that the seat next to you is empty! What opportunities are there on the train for eating? You might want to stock up before you leave, as food on trains can be expensive.
I went on the Indian Pacific from Adelaide to Perth, and found that everyone was very sociable. Go and sit in any of the lounge carriages and it will be easy to get talking to your fellow travellers.