Trains have been cancelled between Cabramatta and Granville in both directions.
An image of the wires obtained by 9News showed them severed.
Replacement buses have been set up, with 14 currently running.
Transport for NSW said the repairs were expected by about 3pm.
Trains will still run via Regents Park so passengers can change at Lidcombe or Cabramatta to complete their journeys.
“Sydney knows through bitter experience how badly a single track failure can affect services across the network,” Minister for Transport John Graham said.
“Sydney’s train system is complex enough without adding a failure due to one idiot vandalising a signal wire.
“Sydney Trains is working as fast as possible to rectify this incident.”
