This is the list I came up with of "foreign" steam on Union Pacific track:
1. TSRR 400 (MAA7): Interchange
2. TSRR 300 ( T&G 28) Interchange
3. TSRR 500 (ATSF 1316 ) Car Shop shuttle
4. TSRR 201( T&P 316) Ferry move with several cars for mini-series "Streets Of Laredo" to Presidio, Texas. Live for lubrication.
5. GVRR 2248: When in daily service on two miles of joint FW&W/ UP main line of the Denton Sub.
6. SP 786: Ferry move from rebuild site to McNeil
7. Reader locomotive for "True Grit"
8. SP 745: Louisiana Bicentennial Train
9. SP 4449: Various occasions
10. SP&S 700: Various occasions
There may have been other "ghost" moves but those are the ones I could immediately recall.