And yet, out of the mouths of Babes...

We have transportation ELEMENTS that can only go AS FAR AS England, Or the next state, or down the road. The direction is less important, it's the range that matters, and costs money.

You select the element that offers the most cost-effective combination of convenience and comfort to acheive your objective.

A bicycle will get you into town.

A car will get you to the airport or, with pre-established infrastructure (gas stations and roads) all the way across the USA on Route 66 (Eventually).

A Boeing 717-200 has a nominal range of 3,800 km. It couldn't cross the Atlantic or even do sea-to-shining-sea, but it's fine interstate or within Europe.

A 757-200 can manage 7,000+ km, close to trans-Atlantic or trans-USA. You folks can get to England, from New York in one hop or from home in two.

A 747-400 will do 14,000 km. Trans-Pacific (Sydney-LA), New York-Hong Kong, or Singapore-London - I still can't get to England in one hop, from here.

Collectively, these ELEMENTS form a transportation SYSTEM that gets us most anywhere, eventually, but it still seems to take 4 times as long as it might!

To design an airplane to take you from anywhere on Earth to anywhere else would be ludicrously expensive if all you wanted to do was visit your Folks in the next State.

Similarly, if all we want to do is go to Mars, we don't need a Saturn-sized bus, and we need something rather more than a Lunar buggy.

Failure to size your vehicle for the mission in hand costs megabucks, or leads to failure. Look at the Shuttle - too big for flying Earthworms in Space, and too small (too low an altitude/cargo fraction) to do a proper job of building a space station.

So when we talk about transportation systems, remember that they are likely to include a number of elements that are finely designed and totally integrated for particular purposes and you select the element you need, rather than having a modular generic system whose modularity costs in money, strength, range, and safety.