Just one little accident that ruptures the tank and just the decompression alone will be like a bomb going off. Add to that the chance of ignition, because hydrogen and oxygen love to make water, and it gets even worse. Meanwhile, diesel is a copper plated bitch to get to explode. It will burn, but not explode like hydrogen. That's the main reason it's been used for so long, safety.
And don't say anything like "It's on a closed track, there is no way it can have an accident." It'll happen. Trains have missed the end of the line stops and crashed into concourses before. Just do this at rush hour and watch the carnage. And if somebody does on purpose as a suicide attack (think 9/11) and you can kiss a lot of innocent people bye-bye.
