Is Sydney’s new light rail line about saving the planet?

It's consistent with what other studies have found. For example, the feasibility study for the proposed Doncaster rail line in Melbourne concluded that 98% of forecast patronage would be diverted from existing public transport services (see Would a rail line to Doncaster really get cars off the freeway?).

Similarly, the feasibility study for the proposed 12 km rail line to Rowville concluded it would increase the share of all trips carried by public transport in the metropolitan area in 2046 from 12.6% to just 12.7% (see Do new suburban rail lines always make sense?).

The proposed Rowville line is forecast to reduce the number of car trips on a typical weekday in 2046 by just 15,000; that's not especially impressive in the context of the circa 12,000,000 trips Melburnians currently make by car each day and the likely $4 Billion or more cost of the line.

None of this means the CBD and South East Light Rail extension is a poor project. As I've noted before (see Does Sydney's new light rail plan make sense?) the key rationale for it isn't to reduce emissions.

Rather, the warrant for the line – which commenced construction last week – is based on traditional transport and planning objectives, particularly increasing public transport capacity and improving the amenity of the CBD. The reduction in emissions is of course welcome, but it's a long way from being an important justification for this, or most, urban public transport projects.