As an engineer, if I were to outline five rules for effective mass-transit, they would be these:
1 - Focus on rails - Buses give one advantage over cars to their consumer, you don't have to park them. Rails, be them light or heavy, still give this advantage, but also give the advantage of not being part of the road network and thus ignoring traffic and stop lights, and this combined with the guiding force of the rails for turning leads to a higher effective speed, often in excess of 60 MPH in what would otherwise be 30 MPH driving territory for at least sections. They also have greater energy efficiency and can be built with a 0 carbon footprint if electrified with nuclear or wind power.
2 - Focus on natural lines - places where you have a very high density of traffic flowing between effective points on a single line.
3 - Ignore the Park & Ride - if people have to drive a car to use your system, they will need to park that car somewhere, and to remove a sufficient quantity of traffic to make rail construction worthwhile you would need Airport-size garages. That level of volume would also create new traffic problems of its own.
4 - Widen Sidewalks - Not technically part of the system, but if people feel comfortable to walk, they will travel further to and from stations, widening the effective radius of any particular station or stop.
5 - Focus on the Inner City - Natural lines, and the heaviest use natural lines, will lie within the city almost by definition, as it is where the highest density, and thus highest likelihood, exist. It is also generally the most pedestrian friendly place and the place where the most utility (potential places to work, shop, play) exist within a certain distance of a station. Don't even consider building to the suburbs until the inner city is covered or the folks in the burbs will have nowhere to go.
6 - The system is for the people, and not the other way around. (This isn't a rule so much as a general attitude.)
Comments, questions, objections, queries, fact checks, discussion of examples, and personal insults are all perfectly welcome!