Thanks Oxbill, I find resin a bit harder to work with because it is a lot less accurate. but as you say, it's the only way to get something unique because of the sheer cost to make te tooling of a plastic model.
Time for another update.
first I had to make the etched windows fit into the resin brakehouse. this meant a lot of fitting, filing and fitting again. it took some time but here it is;

the steps and brake were next up to be made. the steps are cleverly etched so they fold up perfectly. the instructions call for mounting the brake rod next. the problem is: there is no brake rod on either etch

so, I made my own. below is the finished result together with brakehouse and steps.

before mounting, the interior of the brakehouse has to be painted. not an easy job because of the smallness of the interior. here's the result, attached, after some more filing.

this wagon can be fitted with "compensation" which allows one axle to swivel a bit, making sure that all the wheels stay on the track all the time, which looks brilliant

It is based on the saying "right as a trivet(three legged stool)"
Imagine a three legged stool. All three legs will be in contact with the floor, no matter what the terrain. On the wagon it can be translated this way: The waggon should have only three points of suspension. 2 bearings at the ends of one axle and one on the other, in the middle, turning the 2 contact points on the rails into one on the wagon body.
This is the mechanism. Dead simple, the axles rest in the U-shaped holes, and the whole axle can swivel thanks to the wire that forms a hinge

Because the wagon has a brakehouse, I chose to fit it wtha close coupler mechanism there, so it can be used to couple it to M* and other wagons without all fitting them with screw couplers

The running boards are next up, together with gluing the body together. Now it's starting to look finished, but there's still about two dozen handrails to go, which will complete the waggon.

I will hopefully post pictures of the finished product tomorrow.
In the meantime, thanks for watching.
Rick