I don't think this'd work quite as easily as you'd imagine. If you'd literally take a bus, take out its steering and replace its wheels with train wheels, the resulting contraption would have diabolical cornering abilities with regards to the tight curves typically encountered on streetcar networks.

Two-axle streetcars typically don't have a wheelbase much above 10 or 11 feet at most (and that's already pushing it – there's a reason why everybody moved on to vehicles with bogies or otherwise steerable axles), whereas modern buses have a wheelbase closer to 20 ft or so.