Jarrett Walker has made the point in his
Human Transit Blog that buses and trams/streetcars basically provide the same functional service (see
this post of his). Any differences in mobility between buses and trams/streetcars are due to how the service is deployed rather than the difference between rails vs. rubber. For example, busways with dedicated lanes provide basically the same service levels as railed vehicles with dedicated lanes. BRT (Bus Rapid Transit) with separated rights of way provide basically the same service levels as segregated LRT.
LRT basically has two advantages over buses in terms of the provision of service: it has a higher ultimate capacity because the vehicles can be larger and/or can be grouped into trains. And for the same reason it can have a lower labour cost per passenger-mile.
But unless a bus route is bumping up against capacity limitations, LRT won't really improve service levels over what buses can provide. And if your bus route isn't at capacity and you replace it with larger railed vehicles as a way to reduce labour costs then you're also reducing frequency, which means poorer mobility.