(Same reply as on AMR, duplicated here for anyone watching the thread on MR only)
Consider how many trains you're likely to run on the layout then make all the cassettes you are likely to need in one go. I have tried getting matching stripwood from my local supplier when I've decided I need more cassettes for a layout... Can I get an exact match so that the rails will line up perfectly? Can I heck!
At least purchase enough material all at the same time, it will save a lot of problems later.
My Altgarten (German N scale) operates with cassettes, it is only a very small layout, and can only accomodate a single train at any one time, but has enough cassettes to hold six trains.
Aligning the tracks with a single fixed track it is a simple matter of making a little bay for the cassette to fit into. For twin fixed tracks (but could also work for any number) I use a method that I first saw on Brian Mosby's layout at the Pennine MRC show in Uppermill last year. Springs made from paperclip wire can be soldered to either the fixed rails or the cassette rails and these both align and electrify the cassettes.
The method can be seen on this (AMR) thread.
contains details of a few small dioramas and layouts almost all of which use cassettes for the off stage area. Tue Mill uses a sector plate, which is basically a semi fixed cassette and Cotton Lane has just a single off stage track and makes use of a Peco loco lift, which is just like a commercially available cassette when you boil it down.