First select the cars that will service your -on layout- industries.
Usually that will be about twice as many cars as you have car
spots (for empties in/loads out or vice versa).
Second select the cars that you will use as 'alternates' for those
selected in the first step (for variety over time).
Third select cars for run thru service (the cars in the second set
can be part of these).
NOW - apply your Ratios to adjust the overall layout fleet. If you
still have a lot of cars left over then they can be stored to be used
to swap (again to provide variety over time).
Don't forget to consider the "quality" of the cars you select.
Everybody oohs and aahs about layouts where "all of the cars
looked like museum quality/contest winners".
There are some errors in "car fleet decisions" that I notice the
most (as being "common" problems):
1) Too many home road cars. "I'm a Santa Fe modeler - so I'm
going to run almost all Santa Fe cars."
2) Cars/trains that 'always' go to the same industry/yard and
become 'recognizable' over time. (Also swap out the power
used for a particular train once in a while, same for cabeese.)
3) Too many cars that are "exceptions" - cars the owner fell in
love with and justified with the "it's my RR" rule (over used).
4) Too many cars that are exceptions simply due to the
stated era/date for the layout ... such as cars that would
have been seen in that era - but there are too many of them.
(For instance cars that are older. Same thing for cars that
would have been seen - but not in high numbers because
that particular car type/model was introduced only a few
years before the layout's date.)
5) Cars with paint/lettering schemes that don't fit the stated
era/theme. This is actually quite common - for foreign
road cars that the layout owner isn't up on the nuances
of those other roads. I'm a GN guy - I frequently see GN
cars on layouts that say "this is 1953" ... but there are
GN cars that are wearing paint/lettering schemes that
'weren't invented yet'.
Don't be too enslaved by the concept of ratios. Yes those
ratios existed and were real. But on any one day, on any one
railroad, on any one train ... you would rarely get those ratios
if you simply counted the cars and did the math.
In fact, as long as you don't fall into one of the traps above
(especially #1) and your cars are appropriate for era ... your
crews/visitors won't notice.
But they will notice if they keep delivering the same car
to the same industry every op session.
One last comment - I've operated on a lot of layouts that
were too crowded (had too many cars and trains and tried
to run too many trains in a session). I can't remember
ever saying to myself "this layout needs about 20% more"
(cars/trains) ... there's a lesson to be learned here. *G*
The same can be said of the number of operators - with
the exception that you do have times when "you hold an
Op session and nobody showed up". ;-)
Always go to actual photographs as your guide!
- Jim B.