I'm kinda impressed by the number of possible solutions that have been thought of here. In a practical sense though, I have opted for the one I was hoping to avoid, which is to re-work the track and make a zero-gradient section.
You got lots of solutions - but not to your problem because we didn't read carefully what you wrote. And if we did we perhaps didn't believe it.
I don't think the trains in the example I mentioned were sliding. I have now got one to slide - never seen it before - but I had to make a downgrade of over 10% to get any vehicle, powered or otherwise, to slide down. I do believe you on your 2.5% grade but it's a funny old business if anything slides down 0.5%, as you say it does.