Greens NSW transport spokeswoman Mehreen Faruqi says all trains could run faster if signalling was automated — not just trains on the future North West Rail Link.
The rail link team claim their single-deck fully-automated trains will arrive every five minutes in peak times.
But Dr Faruqi, who is a civil engineer with a PhD in environmental engineering, says the reason the trains will be faster is not solely because they are driverless, as the team suggests.
"We can automate signalling and double the number of double-deck trains that can go every hour," she said.
The upper house member has been campaigning for the government to build tunnels 40 centimetres larger on the rail link to ensure they can service double-decker trains in the future.
She said double-deck trains could carry up to 25 per cent more people.
The rail link spokesman said: "One of the reasons single-deck rapid-transit systems carry more people per hour than double-deck trains is that they are quicker, can accelerate faster and stop at stations for less time because they have more doors for people to enter and exit the train.
"It is the number of people carried per hour, not per train, that is key.
"Once rapid-transit is extended to the CBD via the second [Sydney] Harbour crossing, 30 trains an hour will be able to operate to 98 per cent reliability in the peak. At the moment, only about 18 suburban trains [do]."
Dr Faruqi also wants automated signalling on all train lines and for the rail link to be a publicly owned service connected to the Sydney Trains network.
The rail link's spokesman defended building a new line separate from the existing network, saying adding more trains was not the long-term answer.