One of the things I have attempted to model with Lambing Flat is the prototypical movement of traffic across the layout. A major part of achieving this particular illusion is the provision of loads in the wagons as they move through the visible part of the layout. However, I have never liked having permanently loaded wagons, as this restricts what can be modelled, trafficwise.
Back in the '80s I came up with a design for removable loads, which greatly improved the 'illusion' of the vehicles on the layout actually moving goods.
The basic design consists of a timber base designed to fit in a wagon, with a suitable load glued on the top. Uncovered loads, such as coal, were fairly straightforward, a small piece of foam, left over from scenery building, was carved to shape, painted black, then small pieces of coal glued onto the top surface.
Not so simple were tarpaulins! On the NSW railways up till the early 1980s, tarpaulin covered open wagons dominated ordinary goods trains. The tarpaulins were constructed to standard dimensions (the vast majority measuring 24' x 16') at the NSWGR's tarpaulin factory at the southern end of Enfield yard (the building was still there the last time I looked last new year's eve!) They were made from bleached canvas and when new, were almost white. However, they soon weathered to an almost infinite variety of dirty brown/greys.
I model removable tarpaulins by starting with my standard timber base, then constructing a 'former' from card, if I am modelling the very common version with the tarp supported by two 'A frames' (to give the tarps a 'slope' so they would shed water) or a suitable 'construction' of timber offcuts if I am modelling a high load, such as wool or wheat bags. A standard sized tarpaulin is then made from ordinary printer paper. To get the tarpaulin 'look' I first fold it up, the unfold it and crumple it with my fingers. I then stretch it over the former and glue down with PVA glue, making sure that enough space is left to clear the sides of the wagon! Once it is dry I paint the tarp with Tamiya XF-2 Flat White and then attach a few lengths of a suitable brown thread to the sides to represent the tie-down ropes. Once that is all dry I weather the tarp with washes of various browns and greys untill I get the shade I want.
I also use a variety of other loads, some of which are illustrated below.
The undersides of some of the loads available for use on Lambing Flat, showing the basic construction of a timber base with either a bulk load, such as coal, or a tarpaulin streched across the top.
A pair of removable loads on two S trucks. The load on the left represents a nondescript load of crates while the load on the right represents a load of bagged wheat, correctly covered by two overlapped standard NSWGR 'traffic' tarpaulins.
A rake of S trucks with tarpaulins stretched over the standard A frames much used in the steam and early diesel era.
An S truck and an original wooden BD wagon loaded with bags of wheat, properly covered with overlapped tarpaulins.
S, U and K wagons loaded with coal.
A collection of miscellaneous loads. The D truck on the left has a load of Loco ash, made from sprinkling a mixture of plaster dust and black powder colour over a former of foam. The next D truck has a load of sacks, which, if I remember correctly, are UK made from cast plaster and though designed for 4mm scale British wagons, fit perfectly into a D wagon. The plaster was painted with a wash of acrylic paint. The steel S truck has a timber load made by Duha in Europe and imported by etrains in WA. The last D wagon has another British cast plaster load, representing bagged wheat.
Representative of the other types of loads that can be modelled is this cast whitemetal Fordson tractor (don't ask my what it is, I think I bought it from Keith Hudson's Modeller's World about 30 years ago!) loaded on an E wagon. The steel S has a tarp stretched across the wagon, but without the A frames.
Hopefully, these notes will provide some ideas that can be used to provide removable loads suitable for your purposes!