A second line would be confusing. Surely the purpose of the yellow line is to show where you (and your luggage etc) should be when a "fast" train passes.
That was the original intention.....
Now been hijacked by the industry to mean a line not to cross unless you are boarding. The number of times I've seen drivers hooting or
PA▸ announcements about trains about to stop.....no, that's not what these lines were designed for.....