A fault on Auckland's Western Line, which affected thousands of passengers on Tuesday morning, has been proving difficult to isolate, says rail network operator Ontrack.
The signals failure made it necessary to use buses to transport morning commuters.
A back-up system is being used until the intermittent fault can be found.
Auckland Regional Transport Authority describes the fault as unacceptable - a similar fault affected rail commuters last week.
Ontrack spokesperson Kevin Ramshaw says it hopes the back-up system will allow passengers to return home on Tuesday evening if the problem cannot be found.
Mr Ramshaw says Ontrack plans to instal new, more reliable equipment.
