There's no legal limit to the number of passengers you can carry on a train carriage (see http://www.rail-reg.gov.uk/server/show/ConWebDoc.8192).
So Southern will maximise profits by using as few carriages as possible (a train arrived at Forest Hill today with ONE packed carriage!) unless customers have a more attactive alternative means of getting where they want to go....which we don't.
As it says on that site:
Where an operator of a London commuter train service exceeds levels of train crowding specified in the franchise contract, the DfT can require the operator to produce an action plan for providing more capacity and alleviating overcrowding.
Perhaps lobbying the DfT might be something the Forest Hill Society would consider?