How to reproduce the problem create and build A world on 0.7.6 then update to 0.8.0 then create a railroad with the power tracks on the same world you created now close minecraft then reopen it it will show your world click on it it will say generating blocks then it will say saving blocks then it will freeze and crash

On the older versions it will say generating blocks then after that it will say saving blocks after that is done you'll be in your world