Chris, welcome to the forums
Since you didn't mention scale, I'll borrow a leaf from our hosts - an article written long ago by the late John Armstrong. He designed three 4 x 8 layouts in the then-popular scales:
- O gauge, a VERY compact engine terminal with turntable, transfer table and locomotive servicing facilities - more diorama than layout, but still capable of minimal operation.
- HO gauge, a typical roundy-rounder with a miniscule yard and some sidings.
- I misrecall what the other was.
A turntable usually implies an engine house and steam locomotive servicing facilities, which can completely overwhelm a 4 x 8 in any scale larger than TT.
As for, "A bridge." What kind of bridge? A minimal deck girder spanning a creek or a one-lane road? A humongous (and humongously overbuilt) cantilever spanning the Firth of Forth? A poured-in-place concrete spandrel arch on a curve? Or maybe, the bridge on that John Armstrong O-gauger, a highway overpass.
As stated, your question could be re-phrased (different subject) "I want to meet a young lady 165 centimeters tall." I could introduce you to a two-digit percentage of the female population of six continents - and maybe one or two scientists currently visiting Antarctica. For starters, what scale are you planning to model?
Chuck (Modeling Central Japan in September, 1964 - 1:80 scale, aka HOj)