Different standards for Sydney's inner west light rail line and a new $2.1 billion line from the central city to the south east mean trams carrying passengers won't be able to switch between the two.

The standards differ on a range of measures including gaps between platforms and carriages, height and width of the actual track, clearances between the track corridor and its surrounds, and distance between the back wheels of the trams.

The lack of "interoperability" between the two lines will limit the ability to introduce new routes across a broader network, such as one from Randwick to Pyrmont.

Trams running on the 12.8-kilometre inner west line, for example, will not be able to carry passengers the entire way from Dulwich Hill to Moore Park for large sporting events. Instead, passengers will have to switch to trams on the new line at Central Station.

Tram expert and former ministerial adviser Greg Sutherland said the differences had occurred because Transport for NSW had gone to the market for the new light rail project before drawing up the same standards for all light rail lines.

"They are limiting themselves because they have applied different standards. The one standard should apply to every light rail track in Sydney," he said.

While the trams for the new line will be able to run on the inner west tracks to a maintenance depot at Lilyfield, Mr Sutherland said they could not carry passengers on the latter because of the different standards such as gaps between platforms and carriages.

"They are completely stopping their ability to make better use of light rail technology in the future because they haven't developed appropriate standards," he said.

Mr Sutherland said the differences between the two lines in Sydney was in contrast to Australia's original tramways early last century, which had the same standards across Sydney, Newcastle, Melbourne, Brisbane and Adelaide.

"Transport for NSW never drew up a set of standards to apply to the new light rail trams in Sydney," he said.

"They should have embraced the Victorian standard and made it an Australian standard. If you bought a toaster it has to be run on a standard voltage. The same thing should apply to any infrastructure."

But Transport for NSW said operating the new line as a "linked but independent network allowed us to procure the best light rail vehicles and build the line with the most up-to-date advancements".

"This is also best suited to the route conditions and service levels of the new network, without constraints from existing light rail infrastructure," it said.

The transport authority said passengers would be able to "easily interchange" between the two light rail lines at stops at Chinatown-Capitol Theatre and Central Station.

The 67-metre long trams destined to run on the new line would carry up to 450 people, giving initial capacity to move up to 13,500 commuters an hour during peak periods, it said.

Mehreen Faruqi, the Greens' transport spokeswoman who has a PhD in engineering, said the inner west light rail line was already suffering from overcrowding and to " further limit integration and flexibility on the same mode of transport defies all logic".

"While other nations have achieved interoperability between different rail systems such as high speed and conventional rail, or light rail and trams, this government is going out of its way to make sure that even the few kilometres of light rail we have in Sydney are not compatible with each other," she said.

Dr Faruqi said the government had failed to learn from the historical short-sightedness of different rail gauge systems in Australia.

Transport for NSW declined to say what standards would be used for its multi-billion-dollar Parramatta light rail project, which construction is due to start on in 2018.