Just a quick note that I did not touch directly upon in the rules and I can't remember if I hit upon this in a forum thread. This will be addressed in the next version of the rules (and in the sample game as well).
In ORO, players can become bankrupt and the game ends immediately. Companies force their presidents to go bankrupt.
It's worth nothing that even if a company's stock price goes down to $0, it is not bankrupt. As long as they can keep running or purchasing trains, they are in the game. Basically, they stay floating as long as their president can stay solvent.
Hope this helps.
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Designer's Note: I need to clear up the rules on this point as at one point I wanted to make it so companies could disappear without taking the player out from under them. This didn't work out.