Subject: How fast trains could posibly run. What are the limiting factors.

I read in a book that the fastest trains could in the future run at 
apr 4000 km/h (They were talking about vacum tube trains. This trains 
run in a vacuum tunnel) If the trains goes  faster the passangers 
would simply lift off. The poject of such trains looks quit realistic 
to me. Will there be no other problems (the fastest train in that I 
know and that 'll be in use would only run at apr. 400 km/h)

Re: How fast trains could posibly run. What are the limiting factors.