Modeling the Fox River Line is a daunting challenge. The only car that can be readily modeled from a commerically available product in any scale would be Birney car #48, a single door Birney purchased second hand from the Aurora Plainfield & Joliet (Their #101). Other than the 48, all AE&FRE Birney cars had double sets of doors, a design never offered in any modeling scale.
In O scale, back in the early 1960's (I beleive 1962) Ken Kidder imported the Fox River 300 cars in brass, but in the configuration/detailing they appeared as on the Shaker Hts Rapid Transit. With some effort, these cars can be back-dated/detailed to their Fox River prototypes.
While it's nearly 50 years since these cars were first offered, they can still be found. I've personally managed to locate and acquire 6 of the 7 cars in the past 18 months, and have a line on the last one to complete my fleet. All the established and known traction dealers are your first/best bet, then avail yourself of online resources (including eBay).
In the end I averaged paying about $375 per car, including shipping. A brass O scale trolley with power, fully painted (albeit not for the paint scheme you want), this is pretty typical, actually a bit on the low side.
At this point, some people may decide on interior detailing. An interesting note in all of the Fox River / Shaker Hts 300 models I've ever seen,, not one has ever had the most simplest of interior details, the smoker partition wall ! In any event, I'm not a big fan of interiors, to me it's a lot of work that typically can't be appreciated and I personally would rather build another car than use that same amount of time on something that most people would miss anyways. Just a personal choice, neither right or wrong. Full details are available from the usual suspects, QCar, Current Line, etc. Detail as you feel appropriate.
And there you go,, the Fox River Line rides again.,,,
now the hard part,, scratchbuilding the other cars !