What's left of Philadelphia's trolley system was (mostly) preserved because of a tunnel system that couldn't have been realistically converted to bus use. The last major round of car purchases didn't go so well, as I understand it. The cars were heavier than spec and caused problems. Hopefully the next round is better.
All the smart transit nerds hate on building new streetcar lines that don't have dedicated lanes/rights of way. I don't think they're entirely wrong, but they do tend to downplay the fact that even when fighting with traffic, streetcars have faster and smoother acceleration/braking than buses. They're a better ride, and there are capital/maintenance cost advantages over buses as well.
But in any case, in an existing urban area, if you aren't going to tunnel, then good luck finding those dedicated lanes. It can be done, but...
