I have no experience masking rail cars to paint multiple colors. However, I have been painting architectural models for over 30 years. The best tape that I know of and our standard go-to is Nichiban medical tape. We rarely mask to a line, rather we cover the line to be masked to and then cut the tape with a sharp hobby blade. Any broken tip will ever so slightly cause a rough cut knife line/tear rather than a clean cut. Listening to/feeling the clean cut through the body of the blade holder is key to a quality job. If the tip goes "click", out it comes and a new blade goes in. We buy Excel Hobby Blades #11's at 1,000 at a time.
If you are masking a finicky surface, make sure the tape is burnished down well. On a rail car shell a piece of 100 x 250 styrene with the edges slightly eased may be a good tool to use. Then consider a light spray of the color that you masked to create a seal at the mask line. Then apply your next color.
