What are some advantages and disadvantages that come along with mass transit systems?
A good way to tackle this sort of question is to divide it up into topics and think about the advantages and disadvantages associated with these. This gives you a more structured answer.
Here are a few topics to think about:
Environmental e.g.
Energy use
Area of land used
CO2 reduction?
Terrorism/security e.g.
Large numbers of people
Confined spaces.
Large numbers of people e.g
heat generated
toilets
movement en-masse
time effects