To learn how the mod system works, I created this experimental new configuration for the default Citybus that adds a hybrid drivetrain similar to the hybrid buses found in larger cities like Chicago and NYC. This configuration includes a custom eCVT transmission, which has a built in electric motor for boosted torque and "regenerative braking" which slows down the vehicle using the resistance of the motor (which, on a real hybrid vehicle, captures kinetic energy into the battery and reduces wear-and-tear on the friction brake system).
This configuration is especially great on the "West Coast" map, as the increased torque helps when climbing the steep city inclines, and the regenerative braking likewise helps when descending these inclines.
There is a pre-made "citybus_hybrid" configuration, or you can change an existing configuration to hybrid by changing the transmission selection:
Beta Citybus Hybrid Drivetrain 2.1
A custom configuration of the City Bus that adds a "hybrid" drivetrain