There has been for many years a HO weight recommended practice from the
NMRA. NOROP has also a weight NEM. Now you would expect both organisations to come up with a similar standard, however there
is a large disagreement between the worlds largest model railway organizations
on this subject. Both organizations claim weight is important, but their
solutions are considerably different. It can be assumed at least one
organization has got it terribly wrong.
Most modelers I know who model Australian prototypes have found the
NMRA RP 20.1 to heavy to be practical, especially if you use 4 wheel wagons and
steam outline locomotives. You simply cannot make many open wagons heavy enough
unless they are of all metal construction.
The NOROP NEM 302 is far superior, and uses a linear formula of 0.4g/mm
with a +30% tolerance. It can be shown that when a train is in tension, the
side ways force causing a wagon to tip over on a curve equals the pulling force
on the wagon times its length divided by the diameter of the curve. Thus using
a linear formula like the NOROP formula produces the best result for wagons in
tension. The NMRA has made a big mistake with their formula. From a tractive effort point of view the lighter the train the more a locomotive can
pull. However there are limits, the mass of materials models can be made of,
and any formula should take this into account. The NEM 302 formula is suitable
for all well designed models using modern materials. The second limit is if you
want to use automatic coupling, each type of coupler has a minimum wagon mass for it to
work reliably. For Kadee couplers to work reliably about 75g is needed. The NOROP NEM 302 is to light for carriages
less than 187mm (53 H0 ft) to be reliably shunted.
20 Years ago, before I was aware of NOROP NEM 302, I experimented with
different wagon masses and came up with a linear solution, 0.57g/mm. This
allowed Kadee’s to work on wagons longer than 132mm (38 H0 ft). However this
formula makes long passenger carriages to heavy compared to goods wagons of
similar protypical loaded mass. The result was my passenger locomotives could
not haul maximum length passenger trains. My solution was to use 2 formulas. I
now use the lighter NEM 302 for passenger cars, a nominal 0.47g/mm and for my
goods wagons the heavier 0.57g/mm.
