As the sleek campaign train bearing West German Foreign Minister Hans-Dietrich Genscher glided to a halt inside the cavernous station here, a meager band of supporters from his Free Democratic Party began clapping hands in unison to the blaring music of a Dixieland band playing "When the Saints Go Marching In."

Bounding out to the platform, the portly party leader flashed a confident smile for a cluster of cameras and strode briskly toward a makeshift stand where some young followers were passing out campaign leaflets embossed with the party's slogan, "The Liberals Are Up and Coming."

But overhead, a station signpost gave what could be read as a more accurate commentary on the party's condition as the Free Democrats mobilize for the March elections that will determine their destiny: "Do not climb aboard. The train ends here."

After sharing power for 26 of West Germany's 33 years, the Free Democrats now find themselves threatened with extinction in the coming elections, which were called after Genscher ditched a 13-year ruling partnership with the Social Democrats in favor of a center-right coalition with Chancellor Helmut Kohl's Christian Democratic Union.

The switch in allegiances caused many West Germans to see Genscher as "a shameful opportunist" and contributed to his party's decline in the polls. From a high of 10.6 percent in the 1980 elections, the Free Democrats have plummeted to little more than 3 percent support, well below the 5 percent required to hold seats in the Bundestag, the federal legislature.

Seeking to propel his party out of the doldrums and broaden its appeal, Genscher and other top Free Democratic officials plunged into the campaign with gusto last week as they embarked on a 1,600-mile whistle stop tour by luxury train through 27 cities.

Unlike the two major parties, which count on big business or trade unions to provide a constituency base, the Free Democrats have long been plagued by an identity crisis.

They call themselves liberals in the classical sense, somewhat progressive on social matters but faithful toward free market principles on the economy.

The party's 90,000 members are characterized by spokesmen as "predominantly bourgeois, achievement-minded types" drawn from civil service and white collar occupations.

In human terms, that definition translates into a party seemingly composed of accountants, biology teachers, small businessmen and dentists who own second homes on Spain's Costa Brava.

Hoping to break out of the highbrow, middle-class mold, Genscher took time off from his diplomatic duties last week to hoist glasses and press the flesh with the common folk. He found a decidedly mixed reception.

As the 55-year-old foreign minister entered the Brasserie Sion on a back street near the famed Gothic cathedral here, a few ruddy-faced workers gulped down their beers and greeted him with shouts of "traitor, traitor"--a reminder that his jilting of the Social Democrats had not been forgotten.

Inside the tavern, long rows of old-age pensioners were hunched over steaming bowls of pea soup and glasses of the local Kolsch beer, provided by the Free Democrats.

Grabbing a microphone with aplomb, Genscher addressed the elderly crowd about medium-range nuclear missiles in Europe and predicted that 1983 "is going to be the year of disarmament."

Referring to President Reagan's offer to cancel deployment of Pershing II and cruise missiles if the Soviet Union dismantles its missiles aimed at Western Europe, Genscher said, "We do not want to pursue a policy of all or nothing."

"We have to aim for an interim solution that removes as many Soviet missiles and deploys as few American missiles as possible," he said.

When most of the 400 listeners responded with passive nods, Genscher adroitly turned to subjects bound to whet their attention. Proclaiming that students should repay their grants as soon as they begin earning a living, Genscher stressed that despite budgetary strains, "The people who rebuilt Germany have a right to their pensions."

The entire room came to life in a burst of applause and table-thumping approval. Seizing the momentum, Genscher then made his pitch that only the Free Democrats can ensure the moderate policies needed for stable democracy in Germany.

Alluding to the prospect of an ungovernable political situation if the antinuclear, ecological movement known as the "Greens" captures enough votes to hold the balance of power, Genscher recalled the chaos of the Weimar Republic that led to Hitler's rise to power 50 years ago this month.

"This must never happen again," he declared, and hinted to his audience that the elimination of the Free Democrats from parliament could dangerously erode prospects for "democratic and social stability."

When the soup and beer were consumed, Genscher returned to the party's train to compare notes with other officials who had scattered around town to campaign.

"I've observed a basic and positive change of attitude," said Irmgard Adam-Schwaetzer, the party's general secretary. "People are starting to realize that the change in government in Bonn was necessary and in the best interests of the country."

The train rolled through the lush Rhine valley and stopped in Koblenz, where Adam-Schwaetzer and former West German president Walter Scheel alighted to visit a group of housewives at a riverside restaurant.

As the dapper former president strolled into the tavern and bowed while accepting a glass of the local Mosel, two dozen women blushed and giggled with delight.

Scheel also warned of the threat posed by the radical Greens, who he said were unwilling and incapable of playing a constructive role in a parliamentary democracy.

"We liberals have prevented major swings in Germany by allowing a new majority in government to be formed without politics being turned upside down," he said.

The Koblenz housewives, however, seemed unconcerned by the warnings of anarchy. "We want you boys to take nice pictures of us because we've just come from the hairdressers," Wiltrud Heimann, the group's organizer, told journalists.

The day's road show ended in Wiesbaden, where Economics Minister Otto Lambsdorff was assigned to speak to 300 people gathered in small hotel.

As he sought to spell out the virtues of the Free Democrats' program for economic revival, including tax breaks for small firms and reductions in welfare spending, hecklers shouted, "You are the Judas from Bonn."

The crowd managed to shame the hecklers into silence and Lambsdorff was allowed to finish his speech in peace, but the respectful attention was broken when he uttered a conviction that few people share, at least for the moment, in West Germany.

When Lambsdorff declared that he was "absolutely convinced that we will win enough votes to reenter parliament in March," the crowd snickered, with cries of "Just wait and see."