I ride the red, blue and green lines with my LWB 'bent all the time, usually putting it in the space that has no seats designed for wheelchairs. Only once did I see a person with a wheelchair ever using any of those lines. Works well for me.
Mass transit traditionally runs at a loss. Still does, even in such efficient systems as San Francisco and New York. If it charges enough to break even, ridership goes down. The Pacific Electric transit system (red and yellow cars) that ran in LA in the early 1900s was built by land developers who wanted to sell houses farther and farther from downtown. They could transport at a loss and make up way more than that loss in house sales. It only died as it slowed down to less than 12mph because of all the car traffic, and the fact that it all ran above ground. At one time, there were 1200 miles of track in use!
The limit to the length of train is of course the length of the platform, there is room for more cars and one with more open space would encourage more bike riders. I'm seeing more and more bikes on the trains as time goes on. In Moscow, the subway trains run every 1-2 minutes at peak hours to handle the flow, perhaps we'll get to that point here as gas prices rise, but we'll never get ahead of the commuting traffic curve with new subways and roads. Los Angeles just plain started again too late.