It's quite fun picking a journey, it's difficult getting a balance between the time of the trip, the price and whether it's over-night or not. I tried to go to the big important cities but this is still a rather brief trip as I won't be spending much time anywhere, just to see the typical tourist sites and then when I'm older I can perhaps see China in greater detail maybe. For London to Moscow there is about 6 different options, via Warsaw, Cologne, Amsterdam, Brussels or wherever you wish.
Try
www.seat61.com/ for your options, I made the trip myself but I had lots of ideas from there. They do a good trip to India which also looks good and also a nice trip from London right to Singapore and then to Australia, it's amazing how far you can go from your local train-station :P
The difficulty will be booking it and getting times which work with each other but I'll have to wait a few years for that lol.