I went to Sidetrack yesterday and was disappointed. Basically if you don't do Lionel trains, you will pay dearly for it. The Marklin products they carry in HO scale are all German/Euro models that are extremely expensive, and the Trix Z-scale models are limited in variety and also extremely expensive. I didn't price the Lionel stuff, but I expect (since easily 90% of the stock in the store is Lionel products) that those prices are more reasonable. Bear in mind that Lionel stuff is pricey anyway and requires LOTS of room.
I went home, went into my train room (HO) and for the first time in years, ran trains for awhile. It was a very good object lesson to see my locomotives, which cost me 10% of the stuff Sidetrack is selling, doing a fine job and looking as real as can be.
Bottom line: you can do better than Sidetrack. I recommend online sources or Doug's (they can order what you don't see on the shelf, and it gets there quickly).