Need help on "Let Off Some Steam"

I'm doing the Spirit of Steam: Liverpool - Crewe journey, and I'm stuck on chapter 7, "Let Off Some Steam." I swear to god, the devs are trolling us with this one. After making it out of the station and down the line through the next station, you're supposed to stop on the hill and put the train in reverse so you can back the slate wagons onto a siding. But that genuinely feels impossible to me.

They have you pointed downward on a 1% grade, in the rain, with no vacuum brake. Unless the loco break is firmly applied, the train slips forward down the hill every time, helplessly spinning its wheels the other way, until I pass the signal at the bottom and fail the mission. I've tried sanding, I've tried various regulator levels, I've tried applying the brakes on all the individual wagons, applying the brake in the van at the rear, nothing works. If I apply any wagon/van brakes, I can't back up. If I don't apply any wagon/van brakes, I slide down the hill and get SPAD. Is there a specific trick to this? I can't find anything about this specific mission online.