Cleaners in the RMT union working for Mitie at First Great Western struck on Friday of last week.
Bosses want to impose a below-inflation pay deal and refused a demand to change conditions in line with First Great Western rates.
They also refused to end zero hours contracts or grant overtime rates.
First Great Western workers ballot for strikes
RMT union members at First Great Western will ballot for strikes over the use of agency workers and claims of unfair dismissals. The ballot will affect around 2,500 workers including drivers and guards.