Earlier quoted context omitted.
I'm not sure that the roads really support more peak traffic at this point. Rather, we can reduce the traffic and train crowding by building more housing near jobs so that workers need not embark on 50+ mile super-commutes each day. Housing _is_ infrastructure, and it's the piece of infrastructure we're most sorely missing.
Adding extra lanes to a road just increases the amount of people driving on them, instead of easing traffic. So I would bet the same is true of public transit.
It is very true but the point of adding capacity (to any form of transit) isn't to reduce congestion at peak hours (that congestion is bounded by what people will tolerate) it's to allow more trips to happen with equivalent congestion thereby increasing the total number of trips and/or decreasing the duration of the congestion.