While I haven't added interior LED lights to any cars yet, I have added two exterior LED lights (front and rear) to my customized MTH Jersey Central wreck crane as pictured below.
I used bright LED's from Evan Designs (Forum sponsor) to represent "floodlights" for night work. They are powered from the track as I swapped out the stock MTH trucks for MTH caboose trucks with power pick-ups (i.e. rollers).
I also added two micro, push-button On/Off switches underneath the frame (also from Evan) so that I can turn the front and back lights on or off independently, or turn both work lights off while the crane is being hauled in a train.
You may want to use smaller-sized and/or "softer" lights for your work caboose interior lighting. Evan Designs offers many options to choose from!
Good luck with your project, Eric...and please post some pictures when it's completed!