Where does the steam pressure go? Lets say you build a steam boiler and then you connect several turbines to the boiler in series.  What limits the number of turbines that you can connect to the boiler in this way?  In my mind, it seems that the steam still has to escape, so no matter how many turbines are in the way, the steam will still turn all of them in its attempt to escape the boiler.
 A: Yes, the same mass flow of water coming out of the boiler must ultimately pass thru all the turbines.  However, the part you appear to be missing is that with multiple turbines in series, the pressure drop accross each one is reduced.  There is no free lunch here.
The total pressure drop from boiler to ambient (or a condenser in a recirculation system) is ultimately the same.  Generally it is more efficient to harness this power in a single turbine, although that turbine might have multiple stages which you could think of as multiple turbines in series.  Each stage reduces the pressure and temperature of the steam, thereby taking power from the flow.
It could even be worse, depending on whether you have a closed system or are just venting the output of the last turbine into the atmosphere or the creek behind the plant.  In the latter case, the steam could condense and turn to liquid somewhere along the line of turbines.  That would make rather a mess with turbines intended to run on gas.
