[RichD]

A Magnet Marshall is used to measure the attraction between the magnet(s) and the rails with the magnet(s) in the car. If you want to know the strength of the magnet itself you need to use a magnetometer (gauss meter). Magnets from the same source can vary in strength by a significant amount, however the location of the magnet in the car can be more important than the strength of the magnet, therefore Magnet Marshall readings are probably more relevant.

 

[RichD]

I have a magnetometer and they are rather costly. You can buy a Hall effect sensor and make your own cheap magnetometer. A home made magnetometer is good for determining the relative magnetic strength. We used home made equivalents to the Magnet Marshall before those were available, there was one for each track and there was a standard magnet so that they would all read the same.

The device did not tell the whole story, however. Where the magnet was located was also important and we had to change our rules to take that into account. Eventually we got away from using magnets.