Good luck. My cruise ship just two weeks ago was in Skagway on Wednesday. I contacted WPY earlier this year when I realized that 73 would be back just in advance of my visit.
They said that 73 will only run Mondays, Thursdays, and Fridays. Wednesdays are one of the busier days with the cruise ships, so they focus their efforts on the regular trains. There were five ships in port when we were there.
Regardless, train ride to Summit was pretty cool. I don't know what day they opened the upper loop, but there was hardly any indication trains had been on the loop just yet.
Will have to go back and check out the 73.