I'm confident that given the tendency of big projects to massively overrun cost and schedule, capital costs will be much higher than forecast. I'm confident that given the political reluctance to set fares high enough to cover operating costs, the relevant authorities will be scrambling to find other sources of money, which includes dipping into other pots, which includes other components of transit controlled by an authority. I'm confident that the forecasted economic figures provided by boosters are based on the most optomistic assumptions they can justify, and that the results will be lesser.
A lot of money, not necessarily where it will produce the most utility, sunk into one risk.