THE HEART of the controversy in last week's Metro board proposal to raise bus and subway fares is not whether the fares should be raised but how the increase would be applied. Should suburbanites shoulder the necessary increases by paying more for every mile of their long Metro trips? Should the base fare for all Metro rides, short and long, be increased to place the burden equally on city residents who take short trips and suburban residents who go long distances? No one doubts that Metro needs increased revenues, given its very generous workers' contract and the fact that the federal government is about to cut subsidies for the Metro operating budget.

This latest suburban-urban controversy was sparked by a suburban proposal for a reduction in the cost of riding the subway from the current 12.5 cents to 7.5 cents for every mile beyond the eighth. Last week the Metro board asked for an increase in the cost of traveling every mile beyond the third mile from 12.5 cents per mile to 13.5 cents. The issue won't be resolved for a while. But it is already obvious that too large an increase in long-distance fares could cause some Metro riders to return to their cars, causing faithful suburban riders eventually to have to make up the loss of revenues with higher fares. Suburban representatives to the Metro board argue that all riders should be willing to pick up more of the cost of long-distance rides to keep the largest possible number of riders using the system.

The other side of this dispute involves city officials' rebuffing attempts to impose a 10-cent surcharge on bus transfers. The transfers are free now, and a walk by almost any Metro stop will reveal a brisk business in people hunting up unused transfers from people getting off the bus. The idea is to avoid paying any fare. Metro estimates that the dime surcharge for transfers would raise about $2.2 million. City officials sitting on the Metro board point out that city residents need to transfer from one bus to another much more often than suburban riders and thus would pay the lion's share of the 10-cent charges. They contend this would be unfair.

This your-court/my-court game between city and suburb over the cost of Metro has been in progress since the first rail was put down. The suburbs did not want to pony up their share of the construction money without guarantees that the city would not default on its financial obligations to Metro as the construction moved out to the suburbs. But the quarrel is becoming ridiculous, and common-sense and common-interest solutions are fairly plain.

A sliding scale of costs per mile of travel would seem to be logical answer to keeping long-distance travelers on Metro while having them pay their fair share of the costs. The scale would reduce the cost for every added mile traveled while starting with the same base fare for all riders. The bus transfer surcharge of 10 cents, on the other hand, should be adopted outright. It will, in fact, place a greater burden on city bus riders, as has been argued. But the added charge will also help to eliminate the widespread cheating with transfers that is taking money from the pockets of both city and suburban riders. It would not make sense to increase the base cost of riding the bus to absorb the lost money from improperly used transfers. The very small surcharge will help to keep fares relatively low for everyone.