Many of the examples of odd rolling stock in Sweden, lemons, onetimers, comes from the privatisation/state-ification/back to privatisation history. If you look at this page: http://www.svenska-lok.se/motor_main.php?s=21
, you see 30+ of principally identical vehicles. In such case, I put each vehicle in categories, based on year, KiloWatt, topspeed etc, and the final decission on which to include would be made by choosing the most "popular" vehicles from each category. Then, I think every vehicles in the pakset would have a place and a role, especially when shunting is introduced!
Also, I look wether rolling stock are/where well known or unknown, and prefer to choose a littera that many people would recognize.
In the later years, the famous Rc family had its relative unknown mother, Rb, which was built in 6 examplares to experiment with the equipment later installed in Rc. I have included Rb in the pakset due to its role in relation to Rc, and also because the graphics are very similar to the Rc. I intend to make the cost and maintenance cost slightly higher than the RC to reflect the "experimentation" and unstandardized electronics and make it obsolete in the depot as soon as the Rc arrives, which will give it 5 years.
I too think that there should not exist vehicles that in every occasion is worse than any other existing vehicles. However, with the introduction of shunting, I have faith that many vehicles, which in the current simutrans would be pointless, suddently might make sence (as the locomotor-page linked to earlier).
how do you treat vehicles that were built only as a single (or two) prototype?
in pak.CS we have quite a lot of them. Some have been abandoned because they were technological failure, some were improved further, and others for economical or political reasons.
My idea was to either make them significantly more expensive (those that were technologically bad), and to retire them within short time after introduction.
It somehow feels weird to have a one timer in multiple copys on the layout. If an engine was an experiment, it should only exist in one example on the map. If a second hand marked was ever introduced, it would be cool if you could buy that ONE vehicle (cheaply, due to the original owners finnished the experiment). Maybe set a flag in the datfile with "can_only_buy_second_hand=1" along with a "max_quantity=". It would at least greatly add to the immersion to be able to buy second hand vehicles, also in respect to buying other old vehicles (Sweden imported from around year 2000 old diesel locomotives from Denmark, originally built 1960'ies)!
It really is a time investment to create graphics (and also code) for a vehicle, and it really would be a shame if graphics for a vehicle is not needed due to the vehicle appears not to fit in balance wise.