I'm confused. Do you have flickering lights in passenger cars and you want to get rid of the flicker?
What he said. Or do you want to add flicker? For example there have been threads about adding flicker to simulate candle-lights in dining cars or pre-electric interior illumination. Or adding flicker to subway car interior lighting to simulate the intermittent loss of 3rd-rail power.
But presumably you're trying to get rid of flicker and you want to stay with incandescent/filament bulbs (vs. convert to LED). Rather than a game of 20 questions, perhaps elaborate on exactly what you're trying to do.
Some of the 20 questions to jump start the process might include
Do you have an inclusive list of the lighting configurations for the various cars you're trying to convert? Do any have bulbs where screw/bayonet type sockets are riveted to the chassis (so the metal chassis is electrically connected to the bulb)? How much space is available? Command-control track voltage or conventional operation? Are you comfortable working at the component level (resistors, capacitors, diodes), wiring, soldering, etc.
What is your budget?