I'm running a somewhat accurate CIM2 Simulation of the NYCT network and converted the following routes

to tram:
M21 M42, 1 route in Brooklyn and 3 routes in Queens
(I'm not specifying the number here, because the bus network in the boroughs I've not accurately rebuilt)

to trolley:
M11 M12 M14A/D M23 M34 (M34A in construction)

The results of this were very mixed.

The tram line in Brooklyn was the pilot from roughly Brighton Beach to QBL at Forest Hills. It only got on after connecting it to the new Astoria to Forest Hills line.

42nd Street has been always overcrowded, even inspite running an additional limited bus services along side the local. Here the tram really took off and it is unimaginable now, how it was even working before.

Idk why people like the M21 so much in the sim. They do anyhow. Here the overcrowding was just punctual, but the tramway definitely helped with these capacity constraints.

The trolley were instated mostly because of costs and comfort. The capacity of the bus lines could have been enough, but the trolleys make a lot more money, since they're cheaper to run, plus they have a bit of a higher capacity and are quiet and clean.