In reply to DjGreggieP :
BNSF's headquarter is in Fort Worth, TX. But they were largely created from Santa Fe, Great Northern, Northern Pacific, Frisco, SP&S, CB&Q and Frisco, so you've got the southern transcon line from the ATSF main, then a bunch of PNW trackage from the GN, NP and SP&S (the Burlington Northern part), and then Frisco, CB&Q, and ATSF had a ton of midwest trackage.

Red is BNSF rails, pink is trackage rights over SP/UP.
Why is their stuff in Canada? Either leased power, on run-through assignments, or settling horsepower-hour accounts. Run-through or pooled power is a way to reduce downtime. Rather than stop a train where Railroad A hands off the cars to Railroad B and swap locomotives, you just change crews and continue on with the same power. It's faster and simpler, as long as Railroad B doesn't have some sort of unique cab signal system that Railroad A doesn't have and so doesn't have their power equipped with.
Horsepower-hour ties into pooled-power agreements. Say, Railroad A sends four 3000hp SD40-2s onto Railroad B's tracks and they're there working for 3 hours on a run-through agreement. 4x3000x3=36,0000 horsepower-hours, so Railroad B now owes Railroad A 36,000hp hours. Now, to fulfill that, Railroad B sends three 4000hp ES40DCs over for 3 hours, or two 1500hp GP15-1s for 12 hours, and their accounts are even.
Now, typically a run-through agreement keeps accounts fairly even, but you can get into situations where your horsepower-hour accounts get seriously out of balance. The big one that comes to mind was when CSX and Pan Am Railways tried run-through service. CSX was constantly sending C44-9Ws and ES44ACs (both 4400hp) east on Pan Am, but Pan Am had tried to go entirely with 3000hp 40-series EMDs (SD40s and GP40s) for their roster. Whenever Pan Am tried to send them west, CSX almost always turned Pan Am's power away due to the extreme poor condition, so Pan Am's horsepower-hour accounts were always in arrears. Even when they did get to send their power onto CSX rails, the old 3000hp EMDs couldn't rack up horsepower hours fast enough, both due to the lower horsepower and because CSX's better track conditions allowed less time on their system. Pan Am's system was full of 10mph restrictions and derailments, so the 4400hp CSX GEs hung around a long time on PAR and racked up huge hours. CSX eventually ended up selling Pan Am some C40-8s and C40-8Ws both to ensure that their own power wasn't always tied up on Pan Am, and to allow Pan Am to try and settle accounts. Ironically, those Dash-8s ended up back on the CSX roster a few years later when CSX bought out Pan Am.