The idea sounds promising: Emmylou Harris' new backing band, the Nash Ramblers, made up of former members of some of the country's premier bluegrass groups, recording an album on their own with Harris sitting in. But, alas, the record itself is something of a letdown. The playing on Spring Training is hot, and the harmonies raise chill bumps, but the songs mostly country sentiments revolving around drinking and not getting paid enough down at the factory are based on tired ideas. They don't really come to life even when mandolin/fiddle wizard Sam Bush puts the melodies into charging, bluegrass overdrive. There is one stirring Harris-Jackson duet, ''Don't Let Our Love Die,'' but it's not enough to save the day. Sometimes a backup band is just a backup band, no matter whose they happen to be.