Lego train magnet strength
Test setup


Take 20 magnets from my collection
Build a test stand with a magnetically attached bucket containing an adjustable weight:
Designate one of the magnets a reference (#0)
Repeat for each one of the 19 other magnets:


Attach the magnet to the bucket
Carefully add Technic beams to the bucket until the combined weight of the bucket, batteries, beams and the magnet reaches the limit and the assembly falls onto the rug below: 
Measure the weight of the assembly and note it: 

Measure the two strongest magnets against each other to get an estimate for the upper bound
Measure the two weakest magnets against each other to get an estimate for the lower bound


Limitations

The magnets were mostly purchased between 1990 and 2000 and were stored for several years in a sunny attic room and then for a few additional years in a cold basement in various states of assembly (separated, joined in pairs or assembled into a column of several magnets).

The weight of the bucket was increased by adding Technic beams, which weigh 2-4 grams each, thus limiting the precision of the breaking point (but see the next paragraph for a more serious limitation).

The kitchen scale has a precision of about 10-20 grams.

All pairs were measured only once.

Care was taken to construct the bucket in a way that results in the center of mass being as close to the symmetry axis as possible, but the amount of deviation was not measured.

Results

The 19 pairs have resulted in the following measurements (all values in grams):

220 (#1)
240
220
220
220
240
250 (#7)
240
240
230
210 (#11)
240
240
230
240
240
240
250 (#18)
240


This results in an average value of 234 grams of force required to pull the magnets apart (including the weight of the lower magnet)

The two strongest (#7 and #18) together: 260 grams

The two weakest (#1 and #11) together: 200 grams



I'm aware of the inherent imperfection in this test method, but in my opinion, it represents a reasonable compromise between time investment and attained precision. I would welcome any critique aiming to improve it though.