Hard to believe, but its has been a decade since the FRA gave the Ok for transit agencies to use modern, lightweight trains. And yet hardly any rail operators have taken advantage of the technology. But perhaps that is starting to change, with the announcement that California will be trading in its antiquated tank-trains for something much more modern:
California State Transportation Agency (CalSTA) and the California Department of Transportation (Caltrans) and Stadler signed a Memorandum of Understanding (MoU) for the design and supply of four hydrogen Flirt trains to operate in California. The agreement highlights the responsibilities and roles of each stakeholder and will lead to a contract which outlines the procurement of the zero emission multiple units, with the option to purchase up 25 units.
Stadler is the only manufacturer in North America that designs and builds rail vehicles compliant with the FRA AVT Crashworthiness Standards.
AVT means “alternative compliance” trains, i.e. lightweight, so it burns less fuel (unfortunately, the decision to use hydrogen might negate the cost advantages…).
Systemic Failure 
DMU means Diesel Multiple Unit, like the eBART extension to Antioch. So are these HMUs?
They are calling these ZEMU’s.