Well, now we enter in the great 'Foreground/Background/Good Enough'
debate, which as you can image is almost religious in nature, but:
Right up close at the edge of a layout (foreground), especially a chest
or higher-level area, where the viewer's eyes are close to the models,
you need rather detailed, good looking models - a piece of rod and
square of styrene as a outboard motor is no way going to cut it - looks
cheap and toylike, and ruins the illusion. This, BTW, applies to layout
photography also - even worse, as the camera seems to accentuate the
blemishs, errors, and inconsistancies (this part may be psychological,
or just bad lighting :p)
Further back, where the eye's focus isn't so sharp, now you have the
mind filling in the details and 'representations' working fairly well
(a marina maybe a meter or so away, yeah, simple shapes representating
an outboard works - much like representating a forest on a hillside
using teased out clumps of scenicing - crappy up close, OK further
away). This is were you can also use underscale items as 'Forced
perspective', and get away with it.
Good enough applies to items which cross between the two extremes
(meaning here locomtives and rolling stock), which you detail to a
certain point so that it looks realistic, but cut enough corners so
that you can actually get enough cars together to model a train in your
lifetime - yeah, you may miss rivets here, or brake valves here, or
whatever; Not a contest winner but it looks fine running).
Trust me, if something looks bad up close in the foreground, the
viewer's mind won't be filling in details, it will be screaming toy!