As already said, the Glacier Express is superb; we have done it both in
winter and in summer--both equally enchanting. Reservations are definitely
needed in the summer (in the winter, we booked only the night before).
And we, too, went 2nd class throughout our whole 2 weeks of Swiss train
travel a few years ago in June and had uniformly good experiences--usually
cars had a separate glassed-in smoking area or there was a separate car.
Two other trains well worth taking are the Bernina Express from St. Moritz
to Tirano, Italy (and back, if you wish--we have done it this way twice)
and the Palm Express (also is partly in Italy and uses both trains and
postal bus). Both of these we booked just the day before. Usually we had
train cars that were about half empty, unless a tour group came on. Riding
the Swiss trains is super; they are on time, clean, well run. Cheers,
Pat in Baltimore