I was in Kee-wee yeserday, so I took the opportunity to have a look along the line between Koo-Wee and Dalmore.
There's a short bridge over a drain beside Dalmore Rd, then a continuous embankment back in the directon of Koo-Wee until you get to the end of Railway Rd. Fortunately a local farmer was agreeable to me going onto his property to access a series of seven bridges across Cardinia Creek and associated drainage channels. All these bridges have timber piers and 11' openings, although a number have cast concrete slab decks. Those with concrete decks have crossheads made from about 12" x 4" rolled steel channel, but timber piles. Presumably the use of 11' openings and longtitudinal deck is to allow maximum headroom for floods. Then it's continuous embankment parallel to the other section of Railway Rd (accessed from the Koo-Wee-Rup to Pakenham Rd) back to the series of bridges over the Bunyip River and the associated drains. There was no sign of any low trestle as photographed by Lindsay Crow in the Train Hobby books.
I subsequently had a look on http://www.bikely.com
, a Google-earth based site which I use for planning cycling trips, to see an aerial view. By the look of things there is a low bridge about 100m long about 750m in the down direction from the level crossing at Koo-Wee, which I suspect is the bridge in the Train Hobby photos. I'll be back in the area next week, so I'll try and check it out, and go armed with a camera this time!
Given that a series of bridges like Cardinia Creek or Bunyip River is going to take a lot of room, I'd be inclined to suggest a low, 11' opening trestle with about 20 openings (that will make it about 420mm long in N scale) a couple of feet in the down direction from Koo-Wee. That will leave a fair distance to the Lang-Lang river bridge to represent the flat area around Monomeith and Caldermeade.