I don’t know the answer but This is a good question for Mike Pechner.
I don’t think a privately owned railroad could or would block a train from another railroad from being interchanged to it’s final destination.
There would be public pressure and possible lawsuits against SMART from citizens living along the right of way but I don’t think they would succeed. Some communities have tried to prevent hazardous material from passing through major population centers and have them routed through more remote areas but I don’t think they have succeeded.
Oakland is or has tried to block construction of unloading facilities using zoning laws.
Berkeley a long time ago declared itself a nuclear free zone in the 70’s I think and they were turned down because they can’t regulate interstate commerce.
Opponents can delay or halt construction of passing sidings and second tracks even if the right of way is wide enough.
It’s happened in the San Fernando Valley Area to Metrolink, to Amtrak/Metra on the Chicago to Milwaukee route and Long Island Railroad triple track project