The regulation in question can only be found as “additional explanations” in the full version of the generic “code of conduct” regarding the behaviour at SBB train stations. This document can be found here (german only), see Art. 8 related to waiting areas in the table under chapter 2.2.
However even in German, the related current wording of that rule is quite awkward. I guess that “und Schlafen” at the end of the last sentence does also apply to individual persons and is not necessarily related to excessive drinking, but it’s not that clear!
(A much better and clearer wording would be: “Nicht gestattet sind Schlafen sowie ausgiebiger Alkoholkonsum, letzteres insbesondere in Gruppen von zwei oder mehreren Personen.”)