I know you said you didn't want to draw from track power but have you considered the yellow glow LED strips from Miniatronics which only draw 36mA current and have a capacitor to eliminate flicker. I have not tried one but seems like I read somewhere the HO caboose version (#100-YCB-01) could be used for N Scale passenger cars. It measures 1/2" x 5" and you can break off one or more LED segments if 5" is too long for your car. At $22.95 it is expensive but when I finish laying my track I am going to give one of these a try. They also make florescent look versions but since I am modeling 1933 era I am going to try the yellow glow version.
I have read about using batteries but I always have the nightmare vision of being interrupted duriing during an operating session and never getting back to the layout for several days resulting in a whole string of battery lighted cars going dead at one time.
Mike B.
Bill
"Never try to teach a pig to sing. It wastes your time and annoys the pig"