For public transport to be effective, it must be ubiquitous, inexpensive to use and the alternatives (private cars) must be a hassle in comparison.
Four rail lines in a major city like Dallas is far from ubiquitous.
I’m not sure how many lines San Diego has, but I know that when I lived in East County and had to go to downtown San Diego for any reason, I much preferred the trolley. It was cheaper than paying for parking, made the trip only 10 minutes longer, gave me a chance to read and relax, and since I was usually going to the courthouse to cover a story, convienent.
I don’t have strong feelings about public transport, but I think it’s a good thing. San Diego is adding on new lines (currently under construction is one that will connect the west-side of the downtown line with Mission Valley (shopping), the stadium, SDSU and once again wind up in El Cajon) and I expect to see those ridership numbers increase. The easier the system becomes, the more congestion increases (and San Diego seems more crowded every trip I make down there) the better the alternative the trolley becomes.