Fred Jansz asked:
"I'm building a few Sunshine, Speedwitch and F&C resin kits at the moment and wonder what you're using to properly weigh these cars and at what's the best point in the (box) cars to add these weights."
I prefer non-ferrous weights to avoid issues with uncoupling magnets "grabbing" the weight. Stick-on tire balancing weights from the auto parts store, or the more expensive A-Line weights from the hobby shop are a quick source. Other modelers use lead from other sources, such as sheet lead roof flashing discussed on the list earlier this month. Mount the weight low in the car along the centerline.
"I ask this because I have five plastic Branchline WP box cars with nuts (supplied by the manufacturer in the kit) glued directly above the truck center. As result these cars shake and wobble on the track. Guess that's where WP got it's name The Wobbly from..."
The location of the weight is not the issue. Are your trucks properly tightened? Are your wheelsets in round? Is your track in good shape? You have other problems than the location of the weight.