is it so powerful

No it isn't so its applications are limited (or it is incorporated with other energy storage devices to handle high peak loads such as vehicle acceleration)

http://en.wikipedia.org/wiki/Fuel_cell

And a list of some of the latest concept and production vehicles (such as cars and motorbikes) incorporating fuel cells.

http://www.gizmag.com/search/?cx=014429377739529077712%3Alaul7lezdmm&q=fuel+cell&cof=FORID%3A11#1442

The fuel cell concept is still under development with small scale and specialty application production currently in progress (real world testing of the concepts).

There is little need to install fuel cells in trains with third-rail or overhead electrification systems predominating.

However, third-rail electrification is a hazard to trespassers on the tracks, and both third-rail and overhead catenary are hideously expensive to install and maintain.

One plan is to only install third rail or catenary at stations. With regenerative braking, only the net energy loss from "start-stop" commuter operation needs to be replaced at each stop. Fuel cell locomotives have the opportunity to use either reversible fuel cells or separate electrolysis units to capture braking energy as H2 and store it for later use. (Unfortunately, somebody beat me to the patent on this one.) The trains would "recharge" while loading and unloading passengers.

On non-electrified lines, there are also excellent applications for fuel-cell locomotives. For example, when a conventional locomotive is operated in dynamic braking mode, the energy from the traction motors (working as generators) is currently put into resistance grids and thrown overboard as heat. Any saving of this energy would be a win, as 100% of it is lost at present.

The power output of a hydrogen fuel cell is similar to the power output of a solar cell. The more power you need the more cells you need.

The advantage of the cell is that the power is not dependant on anything beyond its parts. If the hydrogen fuel cell electrolozies the recombined water output then the cell can be made to continually run.

I think the space shuttle uses them.