There is an old string on this forum (about two-thirds of the way down the page the last time I looked) entitled "Erie-built truck design" that has a bit more information on this truck. (The roster of which Eries had it that I posted came, I think, from the "Railroad Model Craftsman" article about Erie-builts: early 1980s, sometime: I own a copy, but it is in storage in another country....) The "RMC" article had HO scale drawings.
Idler axles on a freight diesel aren't a great idea, but... F-M wanted to market the same basic locomotive design for BOTH freight and passenger service. I think Eries intended for freight use were typically ballasted so there was enough weight on the driving axles for, um, acceptable adhesion (remember that these were not, by modern standards, high-horsepower locomotives: 2000 in theory, maybe a bit less in practice, since the New York Central seems to have thought of them as 1800 hp units). Putting Eries on, say, a coal drag in the mountains would be silly, but they were perhaps reasonable freight engines (by late-1940s standards) for use on mainly level track: the New York Central seems to have liked them enough to order more (forcing F-M to negotiate with GE to extend the production run).
George Elwood's "Fallen Flags" railphoto WWWebsite has photos of Eries, but I don't know if any show the fabricated truck to good advantage. It also has locomotive diagrams from New York Central (and PRR) which may give useful background information. The "Railway Mechanical Engineer" article from November 1946 (reprinted in "Train Shed Cyclopedia" #64) has a good photo of the truck: if you want to send me your SNAIL-mail address, I can try to make a good photocopy.