Groundbreaking analysis measures public transportation’s impact on the nation’s economic productivity for the first time. Investment in transit can yield 49,700 jobs per $1 billion invested, and offers a 5 to 1 economic return. Investment offers productivity gains long after the short-term stimulati...
Should be noted that this applies specifically for the US. Countries with more developed public transport might see lower returns on (further) investments.
And this study was sponsored by a public transport organisation, so there might be bias.
But I haven't worked through the study to truly cast doubt on anything and even if it was only a 2:1 return, it would still be an insanely good invest.