I bought the game just 2 weeks ago so I probably don’t grasp the importance of the last update and how it improved the fleet management.
But it seems like it should be quite easy to improve even more. Please, add a button Create Vehicle to the Line window and to the Line list. It would create the vehicle designated for automatic replacement in the depot nearest to the line start and assign it to the line.
It would also be helpful to have other similar stuff like “replace immediately” or a “sell vehicle” options (e.g. by right mouse click on vehicles or on the line)