Building on Daniel’s suggestion…
?
Let’s say you would like three cassettes that hold 18 cars each (triple your current six).?
create three staging tracks each long enough to hold 18 cars. Call them RED, WHITE, BLUE for example. ?
Then build 9 cassettes and paint (or designate) three of each by color. ?You will have to break down your train onto the separate cassettes, but then at least it really doesn’t matter how they are blocked since the three cassettes will stay together as a group. ?