I also had a 600k pop, independent, problem-free city but I decided to add some industries to get the Technophile and then some rich people which decreased my pop by about 150k:
The only transport system in this city are the Streetcars which are mostly for the looks since they dont seems to be doing much.
But one day... A earthquake hit right in the middle of my city and now everyone is doomed.
What a sad ending...