Well, thanks to all who've been so kind in offering advice, web sites,
and other information. After serious consideration, we've decided that
for the time being we're going to put this pond/railroad idea on the
back burner, since the location I have in mind really wouldn't support
a pond of the size we're thinking about, considering all of the things
needed, and the obvious eventuality of needing space for Koi. :-)

Our current site would be impossible to get machinery in, so making a
pond of 1000+ gallons would mean that I'd need to dig and dispose of
1000+ "gallons" of soil/roots/rock by hand, which is not something
I'd wish on anyone. I know people do it, but those are ones who are
already addicted to ponds and Koi and can't seem to help themselves but
be gluttons for punshment. I'd rather let machines do my digging.

I do have some additional questions, though, as I continue to dream
about plans for the future.

Assuming that I will eventually have a 1000+ gallon pond and use some
sort of flexible liner; how does one keep roots from encroaching on the
liner and potentially poking through the liner into the water and causing
damage and water loss? It would seem to me that putting a liner is is a
very permanent operation, and that a liner replacement would pretty much
be a very large deal both in regards to hassle and time.

I've been reading quite a bit of the posts in rec.ponds, and have a some
questions on filters. Obviously, a pre-filter and/or skimmer would be
used to keep the pump from seeing large particulate (sticks, leaves,
etc) matter. However, is a veggie filter just a natural form of a
biological filter, or do they serve different functions?

In doing some reading, it seems that especially with Koi, maintaining a
sufficient oxygen level is critical. However, I haven't seen anything
about whether it'd be possible to get too much oxygen in the water
possibly leading to algae problems not due to other issues (overfeeding,
sunlight, etc.) Is there such a thing to worry about as too much
oxygen, or can I waterfall, bubble, and fountain to my heart's delight
and not worry about overdoing it? I'd think that maybe running a
waterfall during the day and then a bubbler at night would work well as
a compromise between the cost of running a waterfall pump 24 hours a
day and making sure the fish had enough oxygen.

Also, in regards to Koi, how much do they really cost for some
small-fry type fish of "pet quality" (rather than show) that will look
nice in the pond, but wouldn't break the bank (or my heart) if a
predator might happen to take a liking to a few. Obviously I can get
some goldfish relatively cheaply from the local pet store, but I don't
see much information on the WWW in regards to what a few starter Koi of
average quality would cost.

Scott