I think "S席" may mean "Special Seat". In Japan at least 1910, "特等" grade was used for the top grade seats (http://parasiteeve2.blog65.fc2.com/blog-entry-1135.html). "特等" > "一等(first grade)" > "ニ等(second grade)", or "特等" > "普通(normal or basic)" were used past (these are also commonly used for seats grades even now). "特等" is abbreviation for "特別等級" = "Special Grade". So S may mean Special.