using transistors to turn LED s on and off with the ground leg of the LED.

Hi all.
I am trying to desing and build an automated model railroad led signal control system using transistors.
Each signal stand has 3 LEDs red, yellow , green.
The three LEDs in each signal stand are wired in parallel to the anode with a common resistor.
This wiring is to accommodate existing model railroad signal controls (mostly manual) which ground the incandescent bulbs in the signal stand, long before LEDs existed.

I have read a number of articles about turning transistors on and off but they do not explain or show circuit diagrams where the led is powered and turned on when one leg of the transistor is grounded and off when the leg is open.
I may be missing something or not understanding the explanation.

My expeirance with electronics is as an electro mechanical diagnostic and repair technican. There is much more to the control system I want to design but I need a transistor circuit diagram controlling the ground side of the led to start with.

Any help will be greatly appicatated.
If anyone is interested in what I am tring to design let me know.
Bruce