This is what happens in the real world.
In Manchester when the ‘heavy’ rail line from Manchester Victoria to Rochdale via Oldham was converted to light rail, usage very rapidly increased to 3 times the previous usage and still rising! (The trams serve the city and town centers whereas the trains called at stations on edge of towns.)
This would be a nice feature to add to the game, the ability to convert existing rail lines for light rail use, allowing connection to tram lines in the cities. This would avoid the need to provide new inter city tram tracks.
The feature would also enable the construction of new light rail routes. Steeper gradients would be allowed on lines constructed for light rail.
This would also enable US type interurban lines.