What are you drawing?
If you are drawing locomotives , rolling stock or structures, etc.,
then draw the items in full scale units - either inches or
millimetres. When you plot, you set the scale in the plot setup to
print out in whatever scale you want.
Even if you do your stock or structures drawings in specific scales,
you can plot them out in different scales by adjusting the ratio in
the plot setup, applying the appropriate constant to do the
Also, if you do your drawing using full scale units you can set
dimensions in the drawing to reflect model dimensions so that when you
plot your drawings, you have model dimensions shown. You can do this
by adjusting ratios in the dimensions setup. The only problem with
this is that your drawing is then tied to one scale if the dimensions
are to be correct. However, there are primary and secondary values
for dimensions, so you could show the dimensions for both 7mm and 4mm
scales if that wasn't too confusing. Or you could set up several
dimension styles for different scales and apply the appropriate style
For a layout drawing, then you will probably want to draw in real
life dimensions so that you can directly relate your layout and track
drawing to the space you want to build it in. So track drawings will
be at actual units - i.e. 32mm gauge is 32 metric units. If you have
a source of track drawings which are drawn to full scale dimesnions,
then you can import them to your actual size layout drawing by
applying a scale factor.
There are lots more options in addition to what I've shown above -
you've just got to try a lot out to see what suits you best.
In Cad design it is common practice to draw all parts at a 1:1 scale.
When making a drawing for production, first you have to scale a part
up or down to let it fit onto your papertemplate. Then you set a
dimensionscale (dimscale in AutoCad) for your drawing.
I use QuickCAD, and assume it's similar to AutoCAD for setting scales. Just
set your drawing scale to 7mm and make the drawing. Save the drawing then
change the drawing's scale to 4mm and save that. If you draw in reverse,
4mm then change to 7mm, the drawing will be too large for the page.