In the run up to putting out my long-delayed "Ghosts of Chicago" ebook, I've been reading the Eastland hearings trying to figure it out. Though they'd been known to get 3000 on the ship, in 1908 the capacity was raised from 2200 to 2400. It was then knocked back down a bit; in 1914 it was fixed at 2252, and a few weeks before the disaster it was raised to 2500 - because they now had enough lifeboats and preservers for that many. The official capacity was set at 2570 - 2500 passengers and 70 crew.
The hearings made a lot about "pilings" on the riverbed that Captain Pederson maintained, through the end of his life, were what caused the ship to capsize. This is part of why I just don't believe that the Fool Killer Submarine was something Frenchy Deneau built and planted himself. Adding MORE junk to the riverbed when it was being investigated would have been a dumb move, and if he had planted it beforehand, he would NOT have been wise to call attention to it.