The Unite union said workers had faced the threat of fire and rehire notices and of having their pay cut.
Chiltern Railway members of the RMT will walk out over the dispute on two Saturday's next month.
The Rail, Maritime and Transport union has been in dispute with East Midlands Railway since April.
Merseyrail staff who are members of the RMT union were expected to walk out from Saturday to Monday.
Members of the RMT union are in dispute with Merseyrail over their latest pay offer.
RMT union members oppose the voluntary introduction of a four-day working week with condensed hours.
Members of the Rail, Maritime and Transport Union walked out on Thursday for a second day of strike action.
Members of the Rail, Maritime and Transport (RMT) Union are due to walk out again on Thursday from 00:01 BST.
Transport for London apologises as commuters face another planned walkout on Thursday.
