Travel itself should be quite ok even regularly, it certainly wouldn't be just siting in your seat for 2 days and looking out of the window, more like a hotel on rails. But I'd very afraid of bureaucracy: entry visa, transit visa, various railway companies and their mess of various offers and discounts… Land trip across Asia means passing through countries like Iran, China or Russia. And given how unkind seems rail travel even within EU's borders (where only the last problems applies, and you can get all schedule informations you need by just typing your destination at
http://oebb.at or similar site) for many people, and not because the length of the trip.
On the other hand, it would be wonderful business opportunity for travel agencies. And much more interesting than just booking the right plane. I almost want it to happen :-)