Given that the current line is heavily overcrowding , this is the only extension proposal that has any realistic possibility of ever existing . Even so it would have to be seen to be effective at reducing overcrowding ( by enabling trains to run more frequently ) rather than adding to overcrowding to have any hope of being built . Because the current line is heavily overcrowded this is considered to be the only extension proposal with any realistic prospect of coming to fruition ; but to have any hope of being built , it would have to be seen to be effective in reducing overcrowding ( by enabling trains to run more frequently ) and not to increase it .