Unfortunately I agree with Adam. Aside from the practical problems that have been thoroughly spelled out in this thread, the biggest obstacle is that Westchester just doesn't want it. Westchester thinks that it will only benefit Rockland (and maybe Orange) but will cost Westchester in terms of land taken and of railroad infrastructure that various NIMBYs will object to.
Besides, all that money that could be invested in that could go to the Gateway Tunnel and the projects attached to help WOH service.
People in Tallmans, Monsey, Blauvelt, Orangeburg, West Nyack and Nyack can go to another station if they are so in need of commuter rail.