Seattle officials have nixed plans for the light-rail system on the new SR 520 floating bridge, despite design specifications that call for pontoons strong enough for a rail line. The transit authorities indicated that beyond cost considerations, technical issues made the light-rail line all but impossible. The City is now studying a combination of toll roads and rapid bus transit to relieve the congestion in the busy corridor.
http://seattlepi.nwsource.com/transportation/355421_520lightrail18.html