They have occasionally allowed it, and last year SP 4449 even double-headed with UP 844 on one or two occasions. But UP spends millions on its own steam program, and to get the most return on that investment, is reluctant to let other steam locomotives run on its rails.
The best way to answer this question about the NO foreign steam policy on the Union Pacific is to purchase and read this magazine. I am talking about the "Steam Today" special issue of Trains Magazine. I found this issue to be a very fascinating read about running steam in our current high-tech and high lawsuit era. This website has more info about how to order it:
http://kalmbachcatalog.stores.yahoo.net/tr3.html
That is a very--very complete and comprehensive rundown on the current state of mainline steam across the United States..
---Daniel
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