The system was going to be used between Harlech and Pwllheli
A hi-tech computer signalling system on a Gwynedd railway line has been postponed - because sun glare means train drivers cannot see the screens.
The ERTMS (European Rail Traffic Management System) uses computers in the train cabs to control movements, such as speed when trains go too fast.
It was supposed to go live around now on part of the Cambrian Coast line from Harlech to Pwllheli, but Network Rail said the launch had been postponed.
A long-term solution is being sought.
The Cambrian Coast Line - which runs from Aberystwyth and Pwllheli to Shrewsbury - was due to become the first railway in the UK to use the ERTMS system.
It is extensively used in other European countries including France, Belgium, Switzerland, Italy and Germany.
The system is intended to increase the number of trains which can run on Britain's railway lines, and improve safety.
It allows signallers and drivers to be in constant communication, and puts the train brakes on automatically if its speed goes above what is safe.
Network Rail spokeswoman Mavis Choong said work to develop the ERTMS system continued to progress, with 98.2% of the work already completed.
"We are currently focusing on addressing safety critical issues identified during the trial run in February," she added.
A Network Rail train driver simulator is used for training
Ms Choong said the company "does not compromise on safety".
"It is paramount that we achieve an optimum level of reliability before launching the system for safe passenger use," she added.
"We are now working with our manufacturer to explore a long-term solution."
One of the challenges too is fitting the system into existing trains, which were not originally designed and built with ERTMS in mind, Ms Choong said.
"Our investment on the Cambrian line plays an instrumental role in paving and informing the way for ERTMS to be implemented in the rest of Britain and we're determined to get this right," she said.