Train drivers and guards at operator First Great Western are to begin voting on Thursday over possible strike action.
Industrial relations have hit a "serious low", the RMT union said.
It highlighted concerns about whether managers had received enough training to drive and guard trains.
First Great Western has previously said it was working with unions to resolve issues, but would not comment further. Balloting should close on 10 January.
RMT general secretary Bob Crow said: "There are safety implications in using managers to undertake operational duties, not least because it is unclear what level of training they have received."