Diggle got hold of the battery, but Diaz was right behind him, and Diggle told Felicity to uncouple his train car from the one carrying Diaz.
Felicity wouldn’t do it because it would have meant Diaz escaping, and Curtis stepped in and did it himself.
Felicity’s focus on catching the Longbow Hunters over recovering the battery botched an ARGUS mission, and her assistance during a train heist when an ARGUS team was attempting to stop the Longbow Hunters almost resulted in disaster when Diaz turned up on the train.