The liability issues would be unreal. You would also have to deal with controling them on each end of the bridge, as well as people walking the tracks just to cross the river. Let a piece of wood fall off a flat car and hit someone and bam------multi million dollar lawsuit.
These modifications would trigger an enviromental review of the whole project (I checked) which could be what the real goal is anyway. Of course BNSF giving up their Renton line adds fuel to removing active lines for trails--was at a meeting last week in Chicago and it is already being used as an example for trail advocates getting what they want.
Next you will hear the trail people asking for this line to be abandoned because they are getting by without it now with detours and we want a trail more than trains. You know the drill.