For Washingtonians who wonder how their troubled Metro rapid transit system will ever pay for itself, this little British colony has the solution.

It is quite simple: take all 3 million residents of the Washington metropolitan area and stuff them into Arlington and Alexandria. That approximates the situation in Hong Kong, where more than 3 million of the colony's inhabitants are jammed into an area of about 50 square miles, and it is just this incredible congestion that is going to turn the subway system being built here into a profit-making venture, like nearly everything else in this entrepreneur's haven.

There is no need to spend a lot of money here extending lines to far-out suburbs like Springfield or Rockville. The 15 stations along the 15 miles of track that Hong Kong also plans to call "the metro" will be handling more than a million passengers a day by the time it is in full service, about five years from now. At about 25c for an average five-mile trip, the subway is expected to pull in a profit on revenues of about $100 million a year and expects to have its $1.3 billion cost paid by the early 1990s.

In this city of public transportation addicts, the subway's growth is expected to take little away from the bustling bus and taxi services or the [WORD ILLEGIBLE] minibuses. This last phenomenon, little yellow-and-red 14-passenger vehicles that some transportation planners see as a possible solution to Washington's traffic ills, have reached their logical conclusion here as a source of high-speed thrills, extortion and profit.

About 4,800 minibuses - outnumbering registered taxis - now serve Hong Kong's 4.4 million people. They will pick you up if you wave your hand and drop you off anywhere along the way to the destination displayed in the bus' front window. The fare averages around 20c, but the secret of the minibus success - one that may not go over big in a city so full of regulatory agencies as Washington - is that the driver can in effect charge anything he wants to.

With a fine-tuned sense of supply and demand, the minibus driver will push the price up for lunch hours and holidays. On hot summer evenings when a few hundreds thousand beachgoers are trying to get home for dinner, fares have been known to climb to $1 or more - fairly steep in a city where the average daily wage is only about $4.

The minibuses have become famous for their hair-raising speed down narrow alleys and around turns in the rush to fill their change boxes. The local Chinese crime syndicates have taken to selling protection to minibuses operation in their turf, and wreaking havoc on non-paying buses that intrude.

But Hong Kong's inhabitants tend to shrug and climb aboard anyway. Their mushrooming demand for public transportation, up to 5.3 million rides a day, assures the subway's ability to proceed with little worry as it reroutes traffic and tears up streets just as Washington's Metro workers are doing.

One important difference is that "Hong Kong doesn't go in for rush hours in the sense that Washington or New York do, with the bulk of travelers commuting to and from work between 8 and 9 a.m. and again between 5 and 6 p.m.," said a spokesman for the Hong Kong Mass Transit Railway Corp. "In Hong Kong, people are on the move all day long. That is the kind of place Hong Kong is.

The system is proceeding in a way typical for Hong Kong, where everyone feels that waiting for long-range returns, with China looming across the border, is absurd. In a way unthinkable for a Washington bureaucracy, the government-owned subway corporation here has written into its enabling ordinance the requirement that it operate according to prudent "commercial principles," meaning it must turn a profit and quickly.

The Chinese, who have no subway of their own, have said little about the new Hong Kong subway that they may one day inherit. But the rest of Asia has been openly curious. Only South Korea and Japan have operating subway systems, and some contractors have been engaged in fierce competition, to win a piece of the Hong Kong project, partly to position themselves for possible future subway contracts in Singapore, Manila, Taipei and elsewhere.

The hard-nosed Hong Kong government administrators have seen no need to do any special favors for their economically ailing mother country; they handed out the largest share of contracts - 31 per cent - to the Japanese, with 27 per cent going to British companies and 26 per cent to Hong Kong companies. Only 2 per cent of the business has gone to American companies, including a fare collection contract to the Cubic Western Co.

In method and design, the Hong Kong subway approximates Washington's system. Its engineers are fighting the same problem of water pouring into the diggings because of the high water table. Hong Kong's harbor is too deep to tunnel under, however, as is being done under the Potomac. Instead, huge pre-cast, sealed tubes will be floated into position and then sunk to be anchored and connected on the harbor bottom for an "immersed tube" tunnel system connecting Hong Kong Island with populous Kowloon.

On land, the subway workers are digging down through the streets to build the huge stations needed to serve the expected deluge of riders.

The noise and the inconvenience is terrible, "But the people here really aren't complaining," said one government official. "If this was done in Britain or the U.S. there would be trade unions and citizen groups all over us. But the people here seem to have a deep feeling this will be good for their children, and they don't bother us much."