"We just don't know at this point whether she will make it through," said Erica Kang, project co-ordinator and an official of the Jungto Society, the Buddhist group caring for her. She has been taking only water with salt.
An attempt by Prime Minister Lee Hae-chan to visit Jiyul on Thursday failed after he was turned away by group members.
Environmental groups say the area is home to endangered species and one of a dwindling number of habitats for the salamander.
Korea Eco-center helped to file an injunction under the name "Salamander and their friends" aimed at stopping the construction on the tunnel in 2003, which was rejected.
South Korean President Roh Moo-hyun pledged during his 2002 election campaign to reconsider the plan to build a new track for the last leg of the KTX bullet train.
Jiyul has held three previous fasts to protest against the project. Her most recent, which lasted 58 days, ended in August last year when President Roh promised to conduct a review of the environmental impact of the plan. A review committee has since concluded the project could go ahead and a court injunction sought by environmentalists had been rejected, officials at the prime minister's office said.
Officials at the Jungto Society organisation said Jiyul's protest had far reaching aims.
"She is making a statement about due process. What good is a plan that does not represent sustainable development?" Kang said.
The high-speed railway project will eventually cut the travel time for the 400 km (248.5 miles) journey between Seoul and Pusan to just two hours from the current three.