The biggest disappointment encountered after arriving here in the late afternoon on the Lyonnaise express from Paris is the necessity of taking another train elsewhere. Because, right now, nothing in Europe can match what you've just ridden.

The Lyonnaise is one of the new French high-speed trains put into service Sept. 28. The French call it the TGV ("Train Grand Vitesse"; literally, the train of great speed), and, ultimatley, it is expected to be able to do 180 mph.

It will be at least a year before speeds like this are possible and two more before the complete new railroad is ready, but inasmuch as the trains themeselves were ready, the French National Railroads put them in service.

When the new tracks are laid and the high-speed line opens, the travel time to Lyon from Paris will be cut in half, from four hours to two. The higher speeds are possible primarily because the French are building an entirely new line exclusively for passenger trains; there will be no freight traffic on it. The only other nation with similar train speeds today is Japan.

For the moment, you can still enjoy the new trains even if their speed is a mere 90 to 110 mph, instead of 180. They come in sets of 10 cars: two electric cars at either end and eight others in between, including a stand-up bar and restaurant car (much like a snack bar), and seating for 375 in first class and 240 in second class.

Outside, the sleek, aerodynamic cars are a combination of orange, gray and white, with orange dominant. Inside, in first class, seats alternate horizontally instead of pulling down. The color scheme in second class mixes blues and greens.

In effect, they are luxurious commuter cars. The projected traffic for 1985 on the Paris-Lyon line is 25 million passengers. It is now, and is expected to remain, France's heaviest corridor for passenger traffic. That is why the French feel they can afford to spend about $1.6 billion on building the new line.

The French National Railroads carry 700 million passengers a year, about seven times the traffic of Amtrak. In France, the railroads have a 28 percent market, as against 4.6 percent in the United States.

You should keep in mind that these new trains do not have the luxury of many TEE (Trains European Express)trains because they will be used for, relatively speaking, short commuter distances. Still, on an early October ride, they even smelled new, and if they are enjoyable for the tourist to ride, the French must be ecstatic over them.

"Right now, you don't see too much difference," said Michel Ponssot, deputy director for the railroad in North America. "Even when you reach the higher speeds (on test track), you don't realize it. I think the main difference that you see is in the profile of the train. It is long and thin, with the windows smaller because of the high speed."

To get a better idea of what the new equipment is like, let's compare these trains with the Lyon to Geneva express, which one has 10 minutes to catch when arriving in Lyon on the 12:10 fron Paris on a weekday.

The Lyon-Geneva express is a drab green train that looks like it's been running forever and almost has been. One of the first things you notice upon boarding is the smell -- the W.C., and that doesn't mean W.C. Fields. uHere, there are "compartments" seating six or eight passengers, with an aisle down one side of the car.

On the new trains, the seating is open, with each row consisting of three seats split by an off-center aisle in first class and four seats and a center aisle in second class.

There are no signs that say "Don't Lean Out the Window" in four different languages in the new trains. In fact, the windows are like those on TEE train -- they don't open. If they did you'd be sucked out the window at 180 mph. The public address system is loud and clear, too, while announcements on the Lyon to Geneva run, like those on other older European trains, are next to impossible to hear.

It is difficult to compare the Paris-Lyon trains to Amtrak ones in the Northeast Corridor of the United States because -- even when Amtrak completes it $2.5 billion improvement between Washington and Boston -- the top Amtrak speed will be 120 mph.

Visually, and in amenities, the new Amtrak trains and refurbished Metroliners seen comparable to the new ones running to Lyon, although you may have a fight on your hands if you say that to the French National Railroads.

A spokesman for Amtrak put it this way: "There's no denying that they've got the frequency [of trains], the speed and a lot of other things. But when it comes down to the actual cars, we're probably a shade better."

When the Paris-Lyon run is fully operational, meals will be served at passenger's seats in both first and second class. For now, you can have a quiche and a bottle of wine in the club car, should you like, or sandwiches and a variey of other dishes.

The quiche and other heated lunches run about $5. Sandwiches cost from $2 to $4.50, whiskey about $3.50 and wine $3 to $5 for a small bottle. You can also follow the French example and carry your own bread, cheese, and wine.

Another new twist for European trains is that only seated accommodations are provided. Passengers who have not taken the precaution of reserving in advance can get a seat up to the moment of departure -- provided, of course, that there are seats available.

The high-speed line itself, still under construction, will begin about 25 miles south of Paris and run to a point directly north of Lyon. You cannot then expect to whiz out of Paris or into Lyon at 180 mph because you'll be running on the old track. The line will be free of tunnels and have only eight major viaducts.

While the Paris-Lyon run will be by far the fastest, it is of course not only high-speed line, in France. The French railroad introduced the first daily train in Europe with speeds of 125 mph in 1967 on the Paris-Toulouse line.