Large cities benefit from public transport. It is simply more efficient to transport hundreds or thousands of people together (trans, trams, buses) than each of them individually (cars, taxis). However, public transportation systems have to match the passenger demand. If there are relatively too few passengers for the public transport capacity, there will be idling and waste of resources. If there are relatively too many passengers, the system will be saturated and become inefficient. This balance between public transport capacity and demand is very tricky, since many cities change their transportation demands much faster than their public transport infrastructure.
There is also a clear correlation between the cost, building time, and capacity of transportation systems. Trains and metros are high cost, high capacity, making them unsuitable for cities of roughtly less than a million inhabitants. Trams and bus rapid transit (BRT) are intermediate: they have less capacity, but they cost less and can be built in less time. These are not only suitable for medium cities, but for routes with medium demands in megacities. The low cost and low capacity end of the spectrum is filled by buses and minibuses. Compared to the high and medium capacity public transportation systems, low capacity transportation systems tend to be slower and less efficient per passenger. It is not possible to build BRT for all transportation routes in cities, there will be much more low demand routes than those that justify the investment of BRT. Can we have some of the benefits of BRT in low capacity transportation systems?
BRT have been very popular because a line can be built in less than a year. Metro lines can take 3-6 years, costing six times more per kilometer. Certainly, metro lines have a higher passenger capacity and average speed than BRT. But BRT are a good option for routes which do no thave the high demand justifying a metro line (or the city budget does not allow for it, better than nothing?). They accelerate transportation over buses because passengers pay when they enter stations, and can enter all at once through several doors when buses arrive at stations. The cost: building stations and the public space for them, along with dedicated lanes. Buses spend much time simply receiving payments from passengers. In Mexico City, some buses even have a dedicated person (commonly underaged) to collect the fares.
It would be too expensive to have dedicated stations for all transportation routes in cities such as Mexico. However, the time lost collecting fares could be considreably reduced with prepaid cards, as it has been done in several cities (e.g. Hong Kong, Brussels, Boston...). In Mexico City, there is a notable lack of coordination between transportation systems. Recently a "city card" was released to pay for metro and BRT. Soon it should be usable for tram lines. We are just changing mayor, who has announced that the chaotic routes of "microbuses" (now transporting more than half of the population) will gradually be transformed into private companies, with benefits for the drivers and passengers. It will be very important that they also integrate the city card to accelerate payments. This will improve the service, reduce traffic, pollution, equal headway instabilities, and even leave digital trails which can be used for different purposes. One option would be to let them accept cash, at least for some time. Just let them charge more for it, to motivate people to get their cards. It is fair, if you use cash, you are delaying everybody in the bus. You should pay for that.