Based on todays technology, the most scalable production technique for producing Hydrogen is by Steam Methane Reforming (SMR) At best it is 65 to 70% efficient. Traditional reciprocating large steam engines might achieve 25% efficiency, but steam turbines typically reach 40% but in special circumstances might achieve up to 50%.
The proposal is for the final drive to be electric, which means we also need to consider electric generator and motor efficiencies, which would be broadly in line the the original Diesel Electric conversion which is in the order of 90%
Based on those facts, and assuming there is no as yet undefined new technology, the proposed engine would have an overall efficiency 75% x 50% x 90% = about 33.5% That is broadly in line with the originals Diesel electric engine efficiency. However you would also need to account for the storage of enough Hydrogen which with present technology might require the volume of a typical passenger carriage to contain the necessary high pressure tanks and supply control equipment.
They would have better fuel efficiency if the looked at Fuel Cell technology to convert the hydrogen which can achieve 40 to 60% conversion efficiencies allowing for the same 90% motor efficiencies that could see an engine efficiency in the order of 54%.
What we don't know is if there are any new technologies that will make a step change in efficiency of hydrogen production or the reverse power conversion.