Search for the following on the Download Station (aka DLS) with the category 'Layouts' selected:
model
trainz
railroadz
I would also suggest searching for the following:
railroad
railway
scale
You should search for each of these with 'title' selected then search each of these
again, only with it in the 'description'.
Also, try searching using the name of a model railroad magazine, the publisher of the magazine, and the name of a manufacturer of model trains, as some manufacturers also make their own track plan books.
It's also best that you search for routes/layouts with 'TS2009', 'TS2009SP4/TS2010', 'TS12', 'T:ANE', and 'TMR17' selected. However, there have been model railroadz made in previous editions of Trainz, but the specific pieces needed to make a route
look like a model railroadz included in T:ANE and up were not available at the time, so the creators had to come up with
other ways to create a model railroadz. The pieces needed to make model railroadz like those included in TMR17 and T:ANE weren't available until as early as TS2009 edition.
EDIT:
If you want some info on how to make model railroadz and some of the pieces needed to make it look like a model railroadz, check out this thread:
http://forums.auran.com/trainz/showthread.php?109444-Making-Model-Railroadz