Being blessed with living in a city with an excellent mass transit system (Toronto), I have to add that Toronto doesn't hold a match to Singapore.
The entire country seems to have the perfect balance between private automobiles, taxis, buses, and trains down to a science. The place is miniscule, meaning there is very little space for roads, period. But, the population is relatively wealthy, and most probably could all afford cars, if they weren't so heavily tariffed. It's a smart move, because there just isn't enough space in the country for that many cars.
But by discouraging the purchase of cars, the government now has an incentive to instill the belief that there's no reason to own a car at all.
And there isn't... honestly.
A single transit card got me anywhere I needed to go. I put $50 on it at the beginning of my trip, and each time I got on or off a bus or train, I walked past a sensor by the door. The sensor picked up the presence of the card buried in my backpack, and calculated how far I'd travelled using the GPS equipped on all buses. Once the fare was deducted, the doors opened. The fare is entirely based on distance travelled between boarding and dismounting points, and it automatically handles transfers at point time and place.
I thought I was spoiled in Toronto, but Singapore was something else. It was some kind of mass transit utopia.
To answer your question on how I'd implement mass transit in a city, it's pretty simple. I'd ask these guys to figure out how much it would cost, then pay them:
http://www.smrt.com.sg/business_solutions/projects.asp
It looks like plenty of other cities and countries think the same way.