Bronze, Silver, Gold.

Data is refined in three steps. Each layer is a little cleaner, and a little closer to a question someone asked.

Bronze

Raw. Every record exactly as it arrived, kept so nothing is ever lost.
Silver

Cleaned. Duplicates removed, errors fixed, and everything reshaped to match the open standards.
Gold

Ready. Tables built for one purpose, such as boardings by stop or lateness by route.

The models read from Gold. If a cleaning rule changes, Silver and Gold are rebuilt from Bronze, which never changes.