Hindsight is always 20-20! Shoulda Woulda Coulda!
If they still had the SP&S, through westbound trains that didn't need to be yarded at Pasco could go around the loop behind the roundhouse, over the Columbia River bridge and then either to the ex NP line via Yakima and over Stampede pass or on down the river on the SP&S. No need to even stop other than to change crews.
It was really a stupid move to let the Frisco gang of terror loose the SP&S! Bob Downing must be spinning in his grave. He championed retaining the SP&S but they wouldn't listen!!!
The other stupid thing they did was to loose the MILW crossing of the Cascade mountains via the Snoqualmie pass tunnel. Lower elevation and a much better tunnel! BN actually owned it from 1980 to about 1986 before tearing it up. I suspect that they could have connected the west end to the ex NP line (Bagley Jct - Selleck area to Kanaskat - Palmer area??) bypassing the Renton street running, for a lot less than they spent re-opening the ex NP Stampede pass line.
This map, extracted from a 1952 NP map of Washington state railroads. Notice that both the NP(shown in RED) and the MILW (shown in ORANGE) had branches between these two points. If topography wasn't compatible for a mainline connection, Landsburg - Ravensdale, or even the old Pacific Coast ROW from Maple Valley might have been possibles.
I don't have any hard $$$ facts to back any of this up, so don't bother throwing stones.