"There were 12 bikes on board and [they] couldn't fit any more," Ms Carter said.
On Wednesday a group of 20 "unexpected travellers" to Wairarapa had boarded the 4.44pm train in Wellington, filling up the luggage carriage.
"The group rang to book but apparently couldn't book over the phone so decided to turn up anyhow as it is public transport after all.
"Due to the luggage compartment being full from Wellington, all bikes - other than the four already loaded - were turned away."
One cyclist was turned away in Wellington, as were people in Petone, Waterloo and Upper Hutt.
"So this week we have had seven general commuters turned away due to a full luggage compartment on two different occasions."
Ms Carter, who has commuted on the Wairarapa line for almost 20 years, has taken it upon herself to find a solution to the capacity problem.
"There has been a server compartment empty for nine years. Why doesn't KiwiRail turn that wasted space into a self-serve luggage compartment, similar to the airport buses?"
She said leaving the framing of the servery car and refitting the interior with luggage racks would probably not cost too much: "If this was set in place today, the issue of the luggage with the 20 unexpected people would be solved instantly and bikes could continue to be loaded as normal ... Happy people getting home with their bikes on the train they normally catch."
Ms Carter said it was already apparent, from her small amount of data-recording, that there was a need for a separate dedicated luggage carriage on certain Wairarapa-bound trains.
"In four weeks of info-gathering we have now had two instances where bikes are turned away.
"It seems getting to Wellington is fine, but coming home on the 4.25 train leaving Wellington is the issue."
The Featherston Community Board would be writing to the Greater Wellington Regional Council Transport Committee highlighting the bike and luggage storage space issues.
They would be asking the GWRC to give consideration to a dedicated luggage carriage to alleviate the problem.
Although Ms Carter has not been taking statistics on the trains during weekends, she has been told - "through several sources, including people who have been turned away" - of occasions on weekends where cyclists had not been able to board.
"It would be sensible to also have a dedicated luggage and bike carriage for weekend bikers coming to Wairarapa, especially during summer."