[Groxx]

I've thought of that as well, but you've got serious problems with air resistance (and associated vibration) at high speeds unless you've got contoured fronts and backs. So you'd have to drop and add cars in the middle of the train... added complexity and you'd need every unit to be powered independently (true, many are, but this would be a requirement).

So picture parallel tracks with a merge-point somewhere ahead (or a few, in case one doesn't work). You've gotta split the train, merge in the new car, and merge it all back together, at moderately high speeds. Not much of a fun engineering challenge, methinks. The top-car probably makes more sense in this case. At low speeds, where all cars can be the same shape, certainly. Just remove / add to the front / end as needed.