Why stop at 3, why not 4? The platforms would have to be extended to hold a longer train, which would be expensive and not practical. And just because it is crowded, it doesn't mean it needs more cars. People just have to wait for the next train. That's all.
the franklin ave shuttle the way it is today is fail...they shoulda just kept the original design with 2 tracks there is absolutely no way any extension of it would happen today considering the way it is built