The reason many cities spend large sums on their public transport systems is that heavy automobile traffic congests city streets and cause [air pollution]?. It is believed that well maintained, high volume public transport systems alleviate this. Many complex factors affect the outcome of spendings in public transport, so success in reducing car traffic is not always assured.
Many different forms of public transport has been developed. Here are some:
As evidenced from the list, Rail Transport systems are highly favoured by many cities.