A GREAT MANY good things have not been invented yet, but the Travielo is not one of them.

It sprang full-blown from the forehead of Ernest Nussbaum, so to speak, a year or two ago, and now the Travielo is a retail reality instead of just a fantasy of convenience.

What does it look like? It looks like a neurasthenic orange crate. What force drives it? The force that drives the Travielo is four C-cell batteries. Why did Ernest Nussbaum invent it, why does Weaver's Violin shop in Chevy Chase and Esty Spear in Bethesda offer it for sale for $645, and why have 24 people already purchased one?

The answer to that lies in the very nature of the violoncello. This most lovely of all the instruments to look at, listen to and play has always had a minor practical drawback inextricably linked to its alluring shape, which is roughly that of a very shapely but rather short person. When traveling with a shapely, short person, few begrudge her her own airplane seat, even at today's prices. But with a cello, you do. Even in a car, a cello takes up the whole trunk, or a seat and a half in the back, causing the dog to grovel on the floor and other riders to comment on the virtues of the harmonica.

Until Nussbaum's Travielo came along, you either left the cello behind, inconvenienced fellow automobile passengers, or bought it a full-fare plane ticket (a cello is too fragile to check as baggage).

The great Piatigorsky, who concertized all over the Americas telling anecdotes and not practicing very much, turned this to his advantage in a rakish way. He always bought two tickets on the plane. One said "Gregor Piatigorsky," and the other said "Miss Cello Piatigorsky." People talked, of course.

The young cellist Julian Lloyd Weber, however, who is based in London, dreads the idea of traveling with his cello. He is always trying to promote the cello ticket as half fare, but it seldom works. On a flight to Finland, he was told by a check-in supervisor that if he was allowed to take a cello on board at half fare, everyone would want to take a cello on board at half fare. On tour to Bulgaria, he had to change planes eight times, each time with an argument and increased expense. Weber's conclusion: "after traveling by air with the cello, playing the Elgar concerto seems as easy as, well . . . catching a bus."

Of course the Nussbaum Travielo is not intended for Carnegie Hall or the Sofia Opera House, nor is it exactly Sophia Loren when it comes to the looks department (see photo). Its great contribution is that it comes apart into pieces that fit into a box that measures 31 by 6.5 by 7.5 inches, a shape that will fit into the overhead compartments of most airplanes.

"The Lockheed L1011 has small overhead compartments, so on that one you have to put it in the closet," says John Holt of Boston, who owns a Travielo, is 60 years old, travels a lot, and likes to keep playing because otherwise "I decline in skill very rapidly." Holt took up the cello at age 50, and wrote a book about it, titled "Never Too Late."

How does it sound, you may ask suspiciously.

It sounds great! Ernest Nussbaum brought his in the other day and put it all together. There is a skeletal body of spruce, with a sound pickup on the bridge and a little speaker in the case, and naturally you wonder whether this rig is going to sound like the tail of a stepped-on cat, or an electric celeste, or be more suited to bluegrass than baroque.

No, though. It sounds remarkably like a cello. And a pretty darn good cello. The spruce skeleton resonates enough to provide a warm, woody tone, and the little Travielo is quite easy to play. Lots of times on a real cello, as you bow a note, the cello sort of just clears its throat instead of singing, in protest against an imperfect technique, a blob of rosin on the string or lack of talent. The Travielo is not as moody. It sounds like it wants to please. The sound comes out of a little speaker, plenty loud, and not at all electronic-sounding.

Ernest Nussbaum is a civil engineer and railroad consultant. He knows about how to make train tracks designed for 60-mile-an-hour freight traffic suitable for high-speed passenger trains, and marketing the Travielo is just a sideline for him. He has not yet recovered his investment of about $15,000, because "sales have not perhaps come as quickly as I had hoped."

Sometimes people say, gee, $645 is a lot of money for a traveling cello. Well, sure, but the price has been slashed from $795, and it costs Ernest Nussbaum more than $400 just to make one. He is now at home in Bethesda, should anyone plan to beat a path to his door.