What are they testing for? How does it work? I read in the Jan 2007 MR, MTL has come out with one.
http://www.trainboard.com/railimages/showphoto.php/photo/37760/limit/recent
Jimmy Low
Kuala Lumpur, Malaysia
The scale test car is a standard weight, used to calibrate the scales that weigh loaded freight cars. As for the scales themselves, the railroad charges by weight for most commodities - and also wants to be sure, for operational reasons, that the total weight of a train isn’t more than the locos can take over the road or the available brakes (air and dynamic) can control on a downgrade.
That’s why any change to the weight of a scale test car is to be avoided at all costs. For example, they are only fitted with hand brakes, since brake shoes applied while the train is in motion wear down - and lose weight.
Chuck (modeling Central Japan in September, 1964)
There was a story about them in RMC.I think the type in the photo were built by Baldwin from 1900 to 1930.Inside was filled with lead shot.Some might still be in service.Walthers also made one. Joe
Chuck
Does the loco pull the freight cars until the scale test car move to determine what is the drawbar pull?
While I understand the purpose, please clarify how it works step by step, if you dont mind.
Thks
Jimmy
It has nothing to do with pulling. They sit it on the scales that weigh the cars to calibrate the scales. I have never seen it done, but from knowledge of calibrating in general, I would suspect that no other car can be on the scale while they are calibrating it.
Happy to explain, since it’s nothing like what you seem to think!
To envision a railroad track scale, if you’ve never seen one, imagine a truck scale (or a big industrial beam-balance platform scale) with rails on the platform. Cars are weighed, either one truck at a time or the whole car. The test car has a known, exact, weight, and is used to verify that the scales in various places on the system agree with the line’s master scale - which is usually calibrated by the local government’s Weights and Measures authorities.
OTOH, to measure tractive effort, an appropriate dynamometer car (which looks like a passenger car with windows and doors in odd places) is placed between the locomotive (s) and the cars of a train. The draft gear is instrumented, and directly measures the drawbar pull as the train starts, accelerates and cruises. Simultaneously, a speedometer trace is recorded. Drawbar horsepower (which may bear very little resemblance to design or advertised horsepower) is determined by multiplying the drawbar pull in pounds by the speed in feet per second, then dividing by 550.
Hope that was helpful
Chuck (modeling Central Japan in September, 1964)
To further clarify:
When a small, tabletop scale is checked, technicians use small 1 pound, 5 pound, 3 pound, and such weights (or metric equivalents) to verify the scale is reading correctly. The weights have an accuracy of something like 1/10,000th of a pound or something like that.
A railroad type scale also needs to be calibrated periodically, and a scale test car is used. The car is usually marked with it’s weight, and it’s weights inside can be removed to change the weight of the car for scale testing purposes. Walthers made an HO scale version of this car, similar to the N scale version coming out next year. Some scale test cars were larger things, and (at least on the Southern Railway) some DID indeed have fully functional air brake systems, complete with air tanks, brake valves, etc. It is unknown to me how various Weights and Measures departments dealt with the issue of wearing brake shoes changing the weight of the car, unless there was a chart showing brake lining thickness to weight.
To measure drawbar pull, a dynamometer car is used (sometimes goes by other names depending on which RR it is). Walthers made an HO scale version of this as well, complete with a “guage” showing how much pull is being recorded.
Brad
Those things look cute. How bout a whole train of 'em? 
Scale test cars are required by federal law to insure the accuracy of railroad scales used to weigh loaded rolling stock and insure that shippers aren’t being overcharged.
Scale test cars were always the last car in any train because they had no airbrake rigging. The scale test car was rolled onto the scale that was undergoing testing, and if the scale didn’t measure its weight within a certain allowable tolerance, the scale had to be re-calibrated.
I think railroads were even required to thoroughly wash the scale test car before use because dirt could affect its accuracy.
As you can see at the following web site, scale test cars were not necessarily the small, special built type.