Well the private UP 8444 + SP 4449 excursion departed Seattle at about 11:20AM northbound today (friday 5-18-07). They departed Tacoma WA northbound at 10:06AM.
They were for a few minutes held south of Seattle at Argo for Amtrak train 7 to do station work at Seattle.
To be on the safe side for TOMORROW (Sat 5-19-07) be ready in Seattle at 10:25AM SHARP.
Currently at 12:35PM I am watching the UP Steam Locomotive Trace which is giving an OLD report of 12:03PM near Edmonds WA. I am worried that the hilly topography is BLOCKING any updated report from getting to the UP website. I believe that it is possible that they MAY have already arrived in Everett because that is only about 15 miles north of Edmonds WA.
This shorline topography is certainly gumming up the works and preventing the internet audience from getting REAL time performance on the northern end of this excursion.
The only thing I know is that the steam train is SCUEDULED to depart south from Everett at 2:00 PM. Seattle downtown is about 30 miles south of Everett so anytime after 2:25PM (or so)may be good for the southbound move.

I hope this helps to give you a rough idea about Seattle times on the northbound leg.
---- Daniel