Hydrogen fuel cells have been The Next Big Thing for so long that you'd assume they'd have happened by now if they were going to to. Part of the issue is the transport and storage of a highly flammable gas, not to mention restrictions on road use in tunnels in some countries (they don't even allow LPG). A lot of that (although not the question mark over tunnels) goes away with rail because you have a more tightly-controlled infrastructure, especially as regards re-fuelling - it may even be that the gas can be produced on-site. All that makes it an attractive proposition for railways and, of course, it obviates the issues connected with electricity (a lot of infrastructure, damage to overhead wires) and the pollution issue of diesel. Potentially, there's much to like.
Interestingly, I remember hearing a fire expert talking about hydrogen quite a while ago. They pointed out that a pressurised gas escapes vertically and disperses and said that hydrogen burns with a blue flame and is therefore visible. By contrast, petrol leaks and spreads and burns with an invisible flame. There's a risk of explosion in both cases, but it's different, and hydrogen isn't necessarily more dangerous.
Obviously, hydrogen leaking from a stalled train in a tunnel would be scary, but trains are less likely to suffer tank damage than cars due to the confining nature of the rails.
