If this were any city but Washington, you might say that "the meter is running" for the District of Columbia's taxi-cab industry.

But since it is a federal crime - prohibited by the annual congressional appropriations act - to put a taxi-meter in a District can another aphorism will have to do: The wheel is turning.

A lot of things are happening to the 8,400 licensed cabs that prowl the city's streets.

The number of licensed cabs is continuing to dwindle.

There's talk of a fare increase.

Changes in the kinds of cars used for cabs are under consideration, along with revised rules on radio equipment.

And there's evidence, just beginning to surface, that cab riding habits may be changing because of Metro.

The impact of the subway system is difficult to assess because, surprisingly, no one seems to have studied the relationship between cabs and subways. That is not Metro, anyway and not the D.C. Transportation Department, though Diamond Cab is working on a survey.

Cab is working on a survey.

There is evidence that within the downtown area - Zone 1 on the taxi map - some riders are switching from cabs to Metro, said a spokesman for Diamond, which is one of the city's big three taxi services, along with Yellow Cab and Capitol.

"It really hasn't affected us that much," said the Diamond man. "There are people who ride cabs and people who ride subways and buses."

But a lot of lawyers who used to shuttle between Farragut Square and Judiciary Square in cabs, have switched to using Metro along with Capitol Hill staffers heading downtown and some regular flyers at National Airport.

Those little $1.10 and $1.45 Zone 1 trips can be profitable business for cabbles, who've aggressively adapted to the city's multiple ridership rules.

The subways ought to be generating some new business as well, says Douglas N. Schneider Jr., D.C. Transportation Commissioner.

"Its a visceral feeling I have," said Schneider, "that if you have a good public transportation system, all the elements ought to thrive." Commuters who abandon their cars to take a subway or bus downtown, ought to depend on cabs for running errands during the day.

At the same time, cabs ought to feed people into subway stops and pickup riders there. Some Virginia cab companies say that's happening at the Rosslyn station.

A 1974 study for Metro said the subway presents "new opportunities for use of taxi-cabs" and recommended installing taxi call boxes at every Metro station.

Cab drivers fear the loss of business will be greater when the trains start running into upper Northwest in the early 1980s. Currently that's profitable - and safe - territory for cabs.

Concern for their safety has led to a proceeding now before the D.C. Public Service Commission to allow cabs to be equipped with Citizen's Band radios. Plenty of cabs have them now, but PSC officials think they have the power to prohibit CBs, if for no other reason than to spare passengers from "ratchet jaw" drivers.

The CB channels aren't supposed to be used for business - other frequencies are set aside for radio-dispatched cabs - so the radios would be for safety and traffic reports.

The PSC is also looking at a finer definition of the kinds of vehicles that can be used as cabs. Right now, explained Commissioner William Stratton, any four-door sedan will do.

PSC is considering allowing vans and station wagons to be used a taxis and excluding some of the smallest four-door sedans. The taxi industry - and cab drivers - have time to comment on those proposals before a decision is made.

Not yet before the PSC is the far more controversial question - taxi fares and possible revision of the zone maps. It's been two years since the last increase of 29 per cent.

Drivers say the cheapest new cabs they can buy cost $4,900 and there are lots of cabs on the street that cost $7,000, $8,000 or more.

All the soaring auto expenses that annoy amateur drivers - gas, insurance, maintenance, repairs - come right out of the bottom line of the woner-drivers.

Most cabbies here buy their cars and the upkeep for them through one of the taxi associations, which offer credit on repairs and gas but frequently charge higher prices than independent shops.

Associations that have radio dispatching service charge drivers an additional fee for that, along with renting or selling the radios for the cabs.

Radio service is so critical that drivers for Barwood D.C., a local company, recently objected to the Federal Communications Commission about a plan to consolidate their radio station with that of the Eastern Imperial association.

The Washington drivers complained that they started losing regular customers when their association and its radio station were taken over by Daniel Smith. Smith is keeping a low profile, declining to discuss his attempt to assemble a number of struggling taxi associations into a single group that'll rival Diamond, Yellow and Capital.

PSC Commissioner Stratton - and plenty of cabbies - see a continuing decline in the number of cabs on the street and the number of jobs for cab drivers.

There are 10,700 cabbies who, as they say, "got their face" from the D.C. Police Department's licensing bureau.

The 8,420 cab licenses issed so far this year is down from 8,800 last year, though the licensing year extends until March. Back before World War II, cab licensing officials recall, there were more than 10,000 cabs on the streets of Washington, and a lot fewer people to ride them.