How may I tie in scenic trains with Rick Steves' Tour???

Can't you do it after or before the tour? Add 2-3 days before or after, and do extra stuff.

Yes, but logistically HOW is the question I am really asking here....

Take a train to the starting point.

Maybe slightly easier at the end of the tour: direct train from Bern to Brig, Glacier Express to the St Moritz area where you can explore and take the Bernina Express, then head back to Zürich. 2 days minimum, not including departure day from Zürich.
Consider flying into Zürich and out of Milan, too. Milan is easy to reach from St Moritz via the Bernina Express.

Thanks...I appreciate the thoughtful and non snarky answer...flying out of Milan is a good idea...

I'd say fly over early to Milan. Take train to Varenna on Lake Como for 1, maybe 2 nights for jet lag recovery. Then early train to Tirano, then either the actual Bernina Express or regular hourly train to Pontresina. Next day, short trip to Samedan and get the Glacier Express to Andermatt. At Andermatt, change to trains to Luzern, start point of the RS Best of Switzerland tour, if that is what you are referring to.

I don't think you miss a whole lot between Andermatt and Brig. The long Furka Tunnel (you see the inside of one tunnel, you seen am all), then a long valley down to Brig. Best parts, IMO, are the Oberalp Pass between Disentis and Andermatt, Rhine Canyon between Disentis and Chur, and the "Merry-go-round" Albula line between Chur and Samedan, including the famous Landwasser Viaduct.

Thank you so much...great idea and appreciate your insights on Glacier Express best parts of the best leg!!