Trains to SF on the Fremont line are already crowded as hell before you even get to Oakland. If you live there and are trying to get to the city, good luck finding a seat, plan to stand for as long as 30 minutes in a crowded cart. I hope they thought this through and are adding more trains so that it doesn't make it even more unbearable.
tl;dr: They need 30-40 more cars. Testing on 10 new ones has found issues (which is the point of testing). Contract says 25 more should arrive by the end of 2017.