Rail campaigners determined to see the Severn Beach line preserved will lobby Bristol City Council urging members not to cut a £138,000 subsidy.
Members of Friends of Suburban Bristol Railways (FOSBR) say First has agreed to run a half hourly service if the council maintains the subsidy.
"Without the subsidy, the service will continue to be hourly," said Cat Hobbs of FOSBR.
The cabinet will discuss a proposal to cut the full subsidy to the line.
Campaigners argue that the subsidies must be maintained if the aims put forward in the Joint Local Transport Plan (JLTP) are to be achieved.