Soaking the sticker with WD-40 will do the job also. And you probably already have WD-40 in your tool set somewhere.
Roy Boy what will the WD-40 do to the paint this is a brand new car.
Joe Gozzo
Generally, if the paint is shiny or if it is lithography it won't harm anything. I do not know about flat finishes, however, as I've never had to try that.
Anything you try should be tested on an inconspicuous spot first, to gauge it's effects.
Your post said tinplate, so I assumed (and we all know what that means) that the cars were painted or lithographed steel. If the cars are new production reproduction cars, the modern paint is pretty tough. But still test first.
BTW, if it is lithographed, don't put any alcohol on it. That lesson was learned the hard way.