Keep it simple:
Sub M_snb()
With GetObject("G:\OF\master.xlsm")
sn = .Sheets("trip mapping").Cells(1).CurrentRegion
.Close 0
End With
sp = Workbooks("Rawdatatest.xls").Sheets("linehaul trips").Cells(1).CurrentRegion
With CreateObject("scripting.dictionary")
For j = 1 To UBound(sp)
x0 = .Item(sp(j, 4) & "_" & sp(j, 5))
Next
For j = 1 To UBound(sn)
If .exists(sn(j, 1) & "_" & sn(j, 2)) Then .Item(sn(j, 1) & "_" & sn(j, 2)) = sn(j, 3)
Next
Workbooks("Rawdatatest.xls").Sheets("linehaul trips").Cells(1, 6).Resize(.Count) = Application.Transpose(.items)
End With
End Sub