Hydrogen is very corrosive, and even when used in "hythane" mixes on thing like transit buses, requires special alloys for blends or mixes above 20% hydrogen content, to natural gas ratios, or for all hydrogen combustion systems in the buses and trucks that I've seen it used in. Storage is also an issue as Zark correctly noted

As to cost, the universes most plentiful element Hydrogen can be very costly to produce, or very cheap, depending upon your "feed stock" and the "energy source" you use to convert water, or natural gas/methane into H2 for combustion or fuel cells...

I saw hydrogen produced from Colorado River water, using off peak wind generation, in the Coachella Valley twenty years ago, virtually for "free"... And, used that "cheap" hydrogen in transit buses, truck engines, and to "goose" natural gas fired "peaker" generators put in after the "crisis" from manipulating electrical markets back when Ahnold was Governator.

Off peak hydrogen generation from good old H2O is another way our old pal T-Boone Pickens was looking at to 'store" off peak "wind" power, and use it to fire up combustion turbines, or fuel cells for added peak electrical power demands.

"Fuel cell" "units" for passenger operations ("FCU's" as opposed to "DMU's"???) powered by hydrogen are being tested in Germany, with what appear to be some good results. And, SANBAG is looking at this kind of technology for the Redlands vehicles.

So while Mouse has some of you chasing your tails with his latest post on the impending doom of these evil trains due to these hydrogen powered airplanes... Whatever you do... Please don't break the valve.....