The train fire test measures the ability of a dust layer to propagate flame in order to safely transport, store and handle the material. Two tests are available:

UN classification 4.2
The powder is filled into an inverted mould with a triangular cross section. The mould is then dropped three times to tamp the powder, then inverted onto an impervious, noncombustible, low heat conducting plate. The mould is then removed, leaving a conical strip of powder on the plate.

The powder sample is arranged across the draft in a fume cupboard. The air speed of the fume cupboard is sufficient to prevent fumes escaping into the laboratory and is kept consistent throughout the test. 1ml of wetting solution is added to the pile of powder 30-40mm beyond the 100mm timing zone. The wetting solution is applied to the ridge drop by drop ensuring the whole cross section of the pile is wetted without loss of liquid from the sides. 

A small flame is used to ignite the pile at one end. When the pile has burned a distance of 80mm, the rate of burning is measured over the next 100mm. It is noted whether the wetted area stops propagation of the flame for at least 4 minutes. The test is repeated 6 times unless a positive result is obtained earlier.

If the time of burning is less than 45 seconds or the burning rate is more than 2.2mm/sec then the substance is classified as Division 4.1. If the burning time is under 45 seconds and the flame passes the wetted zone then the substance is classified as Packing Group II. If the burning time is under 45 seconds and the wetted zone stops the flame propagation for at least 4 minutes the substance is classified as Packing Group III.

VDI 2263 burning number
The VDI test is similar to the UN classification 4.2 test, but testing for burning number rates the results as follows: