Maybe they were being leased to to CSX.

but perhaps BNSF was taking delivery of new units being routed over CP via the Flint Subdivision? I also occasionally see CN units roll through Tacoma on the BNSF, so perhaps a pool power agreement of some kind.

I have a pic somewhere of 2 BNSF units, 1 Warbonnet and 1 Bluebonnet running light through Durand Union Station, MI sometime last year 

While some fo the other explanations are possible, the most likely may be that these are part of the motive power sharing agreements the railroads have with one another. Locomotives can appear on foreign lines far from home as they run off hours owed.

Were they headed into the US from Canada? If so they were brand new units. They leave Erie on whatever train possible, allot of times as the lead power.

Here in the Chicagoland aera I have seen alot of BNSF traffic on the CN lines. Saw a coal drag last week on the ex EJ&E line near round out, and again a freight with CN and BNSF units on the ex Milw/Soo/WC line thru Buffalo Grove. Early this year driving to California I saw a CN units with BNSF units  all the way out west in Kingman AZ.

The BNSF / CSX train was eastbound towards Port Huron and Canada. The two BNSF units were in the lead. They looked as if they were brand new, certainly not much road time, too clean.

If competing major railroads lease locomotives from each other then can I justify having any road name I wish on my layout by simply saying they are leased units? Gotta admit, that BNSF paint job makes for some handsome locos.

It's not exactly "leasing". It called power share.

Say BNSF has a train in Chicago, going west, and it needs 4 units with at least 4000hp. They have 3 Dash 9's available, and there's a CSX unit there... The CSX AC4400 will get used. Now say BNSF puts say, 80 hours on that unit, then BNSF owes CSX 80 hours of 4400hp. Now, say BNSF only has an SD40-2 to loan back where CSX needs it, then the hours get up-rated to 118 hours of use, due to the lower HP rating. Power share is about as common now as interchange of freight cars.

*Disclaimer* I may be a little off my my explanation, but this is how it was explained to me some number of years ago (maybe 6-7 years ago).

There are a number of ways a foreign engine may show up.

1) Rented or leased. This does happen every once and a while when one road is power short. Railways lease engines from various leasing outfits (like Helm, CIT Group etc., and sometimes these rental units are in the original colours of the road that previously owned it, with the roadname marked out and new reporting marks tacked on). There's also plenty of examples of railways directly renting engines from each other. During the 1970s and 1980s Canadian Pacific rented engines from all sorts of other railways to help get through power shortages.

2) Run-through agreements. In some places railroads will have specific agreements to share or pool power in certain areas or on specific trains. Common examples are trains that originate on one railroad and run to a destination on another. Rather than changing out the power when it gets on to the other railway, the engines run all the way through and only the train crew changes.

3) Horsepower Hour (HPH) repayment. When engines are running through in pool service, a lot of accounting goes on to make sure that things are somewhat balanced between railroads. At the end of the day (month, whatever) things need to be balanced up. This usually leads to one railroad loaning an engine (or two or three...) to the other for a specific period of time to balance the books.

I'm specifically discounting dead-in-tow moves where new or secondhand engines are being moved as freight to a new owner.

Note that run-through agreements typically involve specific trains.

Units on horsepower hour repayment may be free-running across the using railroad's system. Josh's explanation of how HPH is calculated is pretty good. If I have two of his AC4400s for 100 hours, that's 880000 horsepower-hours that I owe him.