That old subway token? Keep it!

Mark Ford is an eighth grader in the San Francisco area, where his father works for the federal government. They share what Mark calls an ''unusual'' hobby. ''Not many people know about it,'' Mark observes. They are vecturists.

So is John Coffee, a professor of history at Boston's Emerson College, who explains that vecturistm is derived from the Latin term for passage money. A vecturist, then, is a collector of transportation tokens.

Mr. Coffee embarked on this hobby when he was 14 years old in Washington, D.C., where his father was a congressman. ''I remember Meade Peebles, a friend of mine, started me on it. He had a token from Atlanta, and I remember his saying, 'Well, I'm going to start collecting streetcar tokens.' Neither one of us had heard of streetcar token collectors before. We thought we were the only ones who saved them.''

In January 1949, Coffee became the editor of The Fare Box, a post he has held ever since. The newsletter - ''the world's only publication devoted exclusively to transportation tokens'' - is published by the 800-member American Vecturist Association, which was founded in 1948.

''The average member is more interested in coins in general and just wants to keep up on what's going on in related fields,'' Coffee says.

Numismatists (coin collectors) are lured to tokens, he says, because they are less expensive than coins, and also ''because there aren't many people who collect them, and they're available. Just about anybody can get a collection of about 2,000 different [tokens] with very little trouble.'' Tokens range in cost from 15 cents to $200. ''Just a handful of them are worth more [than $200].''

One factor in the value of a token is its composition. ''The most desirable of all are the hard rubber and the celluloids, which haven't been used since the early years of this century, about 1905,'' Coffee notes. ''The commonest substances are brass, bronze, and something we call 'white metal,' which is an alloy of nickel, copper and zinc.''

Harold Ford, who reports on newly discovered tokens for The Fare Box, says data are widely available on the manufacture of coins, but ''if a bus company issues a new token, you have to stumble on to it, as we did in Decatur, Ill., in early July.''

Ford and Coffee had been driving across the country and intended that day to visit Abraham Lincoln's house in Springfield, Ill. During a stop in Decatur, Ford strolled over to a city bus and learned from its driver that the city line used several tokens.

''I was just amazed,'' recalls Coffee. ''This was absolutely incredible! So we immediately changed our plans, stayed over night in the town and the next morning went and got six varieties that we hadn't known were in use.''

Ford believes that there are ''a good number'' of tokens which are obsolete and not yet known to the AVA, particularly the so-called drummer's checks.

''Drummers were salesmen who'd come from big firms from places like Kansas City, St. Louis, and Chicago with sample merchandise,'' Mr. Ford says, ''and they'd go from town to town soliciting orders. The livery stable in each town would usually have a hack service which would meet the trains and pick up the drummers when they came to town. The hack service would collect a quarter and give the drummer a token good for a return trip from the hotel back to the depot. So most drummer's checks are the second half of a round trip fare.

''These tokens were used in little towns along the major railroad lines in the middle West (Wisconsin, Missouri, Kansas, Illinois, Iowa) during the first two decades of the 20th century. They're very desirable, because usually they were not struck in very large quantities to begin with.''

But the third edition of ''Atwood's Catalogue of United States and Canadian Transportation Tokens,'' over 700 pages long, indicates that despite tokens which have eluded the AVA, there is no dearth of those which have been discovered. ''More than 50 percent of all the known transportation tokens in the world are in the US.'' But there are ''thousands more from foreign countries,'' Coffee says. Most of these have been found in England and Sweden, ''with [East and West] Germany a close third.''