Asking advise on travel plans to see UP 4014 "Big Boy"

Hello, My kid loves trains and one of his life goal is to see UP Big Boy. He kept saying all this facts and thr different trains in the US (I remember him mentioning about BNSF?). I actually dont know anything about trains but seeing him passionate about it really want me to make sure this happens.

Now, we got everything settled (US Visa) and currently planning how to go about seeing it. UP big boy site does not seem to post the schedule.

Any travel itenenary you guys can share that will maximize the "train" landscape across the US. Not sure which state to go. I have been to US but only for conferences (CA and NV).

Originally, we were planning to go Amtrak Sleeper train (Zephyr or Starlight- but prolly starlight bec its 35 hrs vs 51 hr)

Any suggestions/comments/ tips is appreciated. Im planning for 7- 10 days.

Thank you.

For context: I live in the Philippines and flying to US would eat up 15 to 20 hours depending where we land. Ill be going with 7 and 10 years old (his the Train fan) so we will prolly land in CA first bec long flights to East coast will be tiring for the kids.