Veolia Transport has apologised for delays to Auckland rail services on Monday morning that saw 22 services cancelled.
Veolia, which operates the region's passenger trains, says it is working with KiwiRail to establish what went wrong.
The operator says a fault affecting the train radio system occurred between 5.15am and 6am.
It says the recent electrical storm and extremely wet weather conditions may have contributed to the situation.
Veolia says Monday's peak hour services will be on time.