I'm sorry that this reply comes so long after your posted question, Ed, and I can not be sure that the following reply and information will still be of interest to you at this late date. On the chance that my reply will still carry advice of interest to you, or perhaps be of valve to someone else seeking similar information, I can offer the following.
To the best of my knowledge no all-inclusion (i.e. any volume covering many individual manufacturers) pricing guide is available. There are a few selective volumes of this sort, but they address only single manufacturers and quite honestly their listed prices are general inflated. I know that there are price guides available for Varney and Athearn HO scale trains and, of course, Lionel O and HO scale, American Flyer S and HO too, but I doubt that you'll find ones for much else. The guides also have the difficulty necessitating that they seller critically judge the condition of his item(s). Most folks think that their stuff could be a potential gold mine, when in fact it is usually far, far from it. Athearn cars that sold for $5 to $10 forty years ago are typically not worth any than that today and often less. Offered as a large group, their value can drop to no more than to average a dollar each!
It is not advisable to offer collections of model trains as a unit, rather than individually. Collections general draw little interest unless including some rare/difficult to find items. Even then, offering the most desirable items individually will draw far more interest than as part of a collection. If your cars and locomotives are not in at least "good" condition, it is probably not worth even attempting to sell them. The best guide as to the current values of individual cars and locos is gained through watching eBay over the course of a couple of months. Thereby you can gain an idea of what items in your collection are actually selling for.
And finally, offering your items in October, November and early December will potentially bring the highest number of possible buyers and highest prices paid.