Thanks for the assistance. St Margrethen is the first stop after the EC[E$] 198 leaves Germany, briefly enters Austria, and crosses into St Gallen canton, where the saver day pass takes effect for the remainder of the day's travel.
4 Dec Train ECE198 Munich > St Margrethen 0654 - 0909 and EC198 > Zurich 0910 - 1027. DB lists no track and a detailed Google map shows tracks from DE to AT to CH. Pass is valid for all travel in CH. Still buy St Margrethen ticket and show the pass at the Austrian-Swiss border 1.8km from the station?
This December trip is Munich > Zurich CH via St Margrethen CH. DB says it is an ECE train and a "layover" of 1 minute in St Margrethen. Google shows 0 changes. SBB has the same schedule, but lists both trains as EC, a retro, stylish train that crosses borders. DB uses the term ECE, EuroCity Express.
I read suggestions for Switzerland > Austria. Also, there is Italy > Switzerland. However, that requires a train change at the border.
Buy a Munich > St Margrethen ticket in Germany and at the border present the Saver Day pass to the (new?) conductor?