I am Building a Blacksmith Models GWR 59' 6" Steam Railmotor dia Q is there a tool that helps curve the bottom side part of the body am I correct in thinking that is called The Tumble???.

There probably is a special tool David.  I managed to do the tumblehome by rolling the side on a wood dowel.  I use the ends as a template to check that the shape is right.

This is my Blacksmith MR Milk van:




There might be some useful info here:

http://yourmodelrailway.net/view_topic.php?id=12498&forum_id=150

If the sides are flanged be careful that you don't cockle them when bending.

John

You are almost right with the name of the area concerned. It is the Tumblehome.  In the past I have used a wooden moulding like a round topped skirting board or similar. In fact the rounded edge of a  worktop does quite well. Do it in stages and keep checking as you go.

I always like to see how others have tackled a particular model.  I came across this:

http://www.scalefour.org/forum/viewtopic.php?t=1415

My kit had lousy castings (most kits seem to suffer from this I've found) and I had to scratchbuild the steps.

John

Rolling bars are the tools you need. Small diameter gives sharp radii, larger diameter shallower. Woks well with thinner metal (10 thou'). They need a softer surface underneath, otherwise the metal will not readily curve. MetalSmith in the UK have a geared 3-bar roller, very expensive. 

In lieu of rolling bars, copper pipe of various diameters from the hardware store plumbing section and an underlay of dense foam (Michaels) works adequately. I got a set off my local plumber and his collection of copper pipe off-cuts. The more resilient and thinner the foam, the shallower the curve. Why kit manufacturers can't put the proper tumblehome in beats me. Comet coach sides always came with this done.

You'll need a 1.5"-2" diameter pipe for a tumblehome, thin foam, and some sort of jig to keep the side fixed, the bar straight and a stop to prevent the pipe rolling onto the vertical part of the side. Or you can just eyeball it.

Nigel