Here are two useful sites for the goods wagon enthusiast.
Firstly, “Goods & Not So Goods – An overview of railway freight operations for modellers.” This labour of love is packed full of interesting material. The site is often an overview, and more detailed information may be found elsewhere, but for a comprehensive view of UK freight operations (and a lot of other things as well) it takes a lot of beating.
Lots of neat sketches, too, including this useful one of how to tie down a container, and a less useful one of a live fish (!) container.
Secondly, Paul Bartlett’s (no relation, I think) encyclopedic site with photographs of almost every BR goods wagon from the 1960’s to privatisation, plus surviving stock from the Big-4. Lots of ideas for liveries and detailing, and prints and downloads are available if you want a more permanent reference. There are lots of Paul’s other photographs there, too, from BR road vehicles to Australian wild life.
Have a browse here.