(in no particular order)
Reduce the size of homes and get rid of minimum parking requirements.
This is important because more dense cities with less parking everywhere allows you to reduce the prices of properties and makes public transport cheaper and more effective per person. This lets you justify better and bigger public transport systems that can be used to go more places with more frequent trains.
Increase public transport methods and density.
While having just one transit system is better than nothing, it is crucial that multiple systems are developed. What happens if train drivers strike, or traffic is higher than usual on one day? (Remember that buses are usually still affected by traffic.)
Build more, faster, more frequent intercity train lines.
This significantly reduces motorway congestion and helps reduce pollution and CO 2 emissions by a LOT. This also helps people without cars (which we want to be a bigger and bigger portion of the population) to still get around and visit everywhere they could want to, often for cheaper (cars are expensive!)
Change policy to be in favour of public transport.
Currently quite a lot of governments (especially in North America) favour cars over other methods of transport. This means that people are more likely to use cars for journeys they're completely unnecessary in.
Encourage active transport.
Why drive when you can walk or bike? Building bicycle infrastructure is the single MOST cost-effective way (as of 2024) for a government to reduce carbon emissions. E-bikes are also, in some circumstances, the most carbon-efficient form of transport possible.
Discourage the use of huge vehicles
Think about it: does the average American pickup truck actually need to be so big? Why can the French get away with their little Citroen Amis? Smaller cars are very good at significantly reducing the negative impact that cars have on our societies.