As I recall, the West side of the Columbia River, South of Interstate 90, is all part of the U.S.Army's Yakima Firing Range. I'm not familar with the area South of Wanapum Dam, however I do know the Army has limited the access to the area around Boyleston Hill and Tunnel. Coming in from Kittitas, you have to call a phone number posted at the gate and speak to the rangemaster. Sometimes it's yes, and sometimes no.
As the Milwaukee Road R of W is now the John Wayne Iron Horse Trail, no motorized vehicles are normally allowed on the trail. ( Humvees excepted ). On my only visit to the area in 1995, I drove my pickup all the way from Boyleston down the Saddle Mtns. grade to Cohasset, where a great view of the Beverly bridge was to be had. With the motorized vehicle restrictions now in place, you would either be on foot, or on horseback.