The train from Chengdu to Lhasa departed at 20:59 PM on the 19th yesterday despite the bridge collapse; the same applies to the train from Lhasa to Chengdu. So there is no need to worry the train ride between the Chengdu and Lhasa.
more info: http://blog.chengdutolhasa.com/2010/08/chengdu-to-lhasa-train-runs-as-well_20.html