I posted a story on this in November when 18 inches of rain fell. Since then the weather has continued very wet, and the full extent of the damage to the infrastructure is immense.
I can already see one political issue emerging. A large portion of the main entrance road washed away and is now the river. The build-around route would require construction in the Mt. Rainier designated wilderness. That would be clearly illegal under the Wilderness Act.
Story in the Seattle Post-Intelligencer. By Debera Carlton Harrell
Other national parks in Washington were damaged too. So was Glacier NP in Montana by the same storm.
Mt. Rainier NP flood page (a NPS web site).