Whats the cheapest way to travel Asia??

No. You are dreaming if you think you can do it. Just food and hotels are going to eat up a lot but the trabnsportation will drain you. You didn’t say how long you were intending to do this so I assummed three days each. That alone is about 500 pounds. Taking a freighter is about the only way to do it, but it will add time to the trip.pp

Since you mention rail pass, I assume that your airfare is not included in your budget. In this case, domestic travel is very cheap and easy in most Asian countries. In China, train is extremely well developed and cheap, especially if you are not afraid of traveling in 4th class (hard seat). Thailand’s bus and train system is also good and cheap, and so is India’s. Japan is in a different league, being a lot more expensive for tourism, but it is not impossible to add it to your list of countries if you are a frugal traveler. Food is cheap too, as long as you eat in small street markets (the best food by the way, especially in Thailand and China) and stay away from western style restaurants.

Oh, of course, this works only if you become a couchsurfer, the best way in my opinion to discover a country and its people. You are adventurous, open, respectful of differences? Try it! You will make friends all over the place, and you will long to see them again. I guarantee you.