I've just bought "Make it run like a Dream: Rolling Stock".  It's an excellent book and a "must have".  I model Danish railways in the late 1960s (the very end of a very long steam-to-diesel transition period for the state railways), so not everything is relevant to my situation but there is more than sufficient meat to make the book a very good purchase.

The last chapter in the book on car weight is thought provoking, and as Joe Fugate suggested, likely to cause some controversy.  So, let me add a footnote to the controversy.

In comparing the weightings given by RP20.1 to NEM302, I think NEM302 is slightly misrepresented in two aspects:

(1) RP20.1 is based on body length whereas NEM302 is based on length over buffers.  I checked what difference this makes to two of the most common UIC-standard freight cars of the late 60s: a 2-axle Gs (covered freight car) and a 2-axle E (open gondola).  It adds about 10% to the length you need to use in the formula but obviously four small plastic buffers add practically nothing to the weight of the car.  So, part of the lightness of the European standard can be explained by the fact that our cars use buffers.

(2) The NEM302 standard of 0.4g/mm is the recommended minimum weight.  The standard further states that the minimum should not be exceeded by more than 30%.  In other words, NEM302 provides an interval of acceptable weight rather than a single value.

In general, European manufacturers of model cars do a poor job at meeting NEM302.  I just did a very unscientific sampling of 21 cars (freight and passenger) and only 7 cars were in the permitted range, 10 were too heavy and 4 were too light. 

My approach is to only deal with the too light cars and then only bring them up to NEM302 minimum.  As long as I do so, it doesn't appear to cause any problems and I'm generally weary of too much weight.  The reason being that following Danish prototype practice of the late 60s, generally trains must be hauled by a single EMD diesel locomotive.  Fortunately, we never ran trains the length of US trains and anyway my stations will only take trains up to about 12ft (and most trains are shorter).  Nevertheless, I have some locos that struggle to meet expectations so this is where I have applied extra weight.

Stay safe

Hans Peter