Network Rail is conducting trials on battery powered trains. They are referred to as IPEMU (independently powered electric multiple units!).
This could be a solution for the Marsh Link Line as the trains would be hybridised standard electric trains and so would be able to run as normal on the electric network.
This trial is not looking at the use of the fuel cell instead of batteries although fuel cells may well prove to be a more effective solution than batteries in the longer term. The results of the trial will be known later in 2014.
The link to the story on Network Rail is http://bit.ly/1grqFzx