The Lenovo ThinkPad P50 Mobile Workstation does not support charging into the laptop through the Thunderbolt 3 based USB Type-C port.
That means if you plug a USB Type-C based docking station like the Plugable USB-C Docking Station into the Lenovo ThinkPad P50 Mobile Workstation using a USB Type-C to Type-C cable, you won't be able to charge the laptop.
We can see this when using Granite River Labs USB Power Delivery test software where the Plugable USB-C Docking Station offers three power profiles 15 watts (5 volts @ 3 amps), 36 watts (12 volts @ 3 amps), and 100 watts (20 volts @ 5 amps) to the Lenovo ThinkPad P50 Mobile Workstation.
However, the Lenovo ThinkPad P50 Mobile Workstation requests zero current to be pulled from the Plugable USB-C Docking Station.