Us Really Old Heads remember BN as being an early modern merger: GN & NP (the "Northern" part), SP&S, and CB&Q (the "Burlington" part), with probably some other minor remnant companies surrounding those from ancient days. Essentially, it made the "Hill Lines" official.

Later on, of course, they added Santa Fe after SP was prohibited from merging with it. Not sure, but BNSF might also have cherry-picked a few useful bits of the Milwaukee and the Rock Island during their liquidations. The SPSF failure was probably the death knell for SP, though it took many more years for the axe to finally fall.