I noticed two things, the centre right brake hanger was very close to the railhead which was due to the keeper plate being bowed. I thought maybe something's stopping the keeper plate from sitting flat against the chassis but upon checking, everything's good and packed away in its place. I think it might partly be due to the staggered retaining screw arrangement where there's two on the ends and one in the left middle, opposite the brake in question. I couldn't bend the plate back to shape. Maybe some hot water treatment might have helped. On closer inspection of the previously mentioned hanger there was plastic swarf and a flat spot. I filed it down and put the keeper plate back on and there was a huge improvement with no slipping let alone stall on a 1in40 curve and a mixed load behind.
I've only just started using the model and can't imagine I've created this bow or the flat spot in one or two evenings of fixing the loose wheel issue and subsequent minor testing. So as I said it might be confined to my model, but, if you've done mods like removed pony truck spring or added weight and still find you're 32 a little breathless, it might be worth a quick visual check on that brake assembly.
With the bow, although pronounced more to the right, it appears along the length.
Notice the swarf and flat spot.
I never took a photo when the brake was touching the rail but in this pic you can see how much I've filed away to similarly match the clearance of the left hand side and note if you can imagine the original fully round shape it would still be touching the rail.