Metro subway riders will have their first chance Monday morning to go one-on-one with Farecard, the magnetically encoded, computer-controlled ticket that will be the only way to enter and leave the train system after it expands on July 1.

Farecard vendors, 4-by-8 foot machines equipped with flashing green and red lights and white buttons, will be dispensing those tickets for any amount between 40 cents and $20 at the seven operating Metro stations between Rhode Island Avenue and Dupont Circle.

Traditionalists who do not want to be part of the computer age can continue throwing 50 or 55 cents (depending on whether it is rush hour) into the fare collection box. Riders who use transfers to and from the bus system must continue to use the fare box until July 1.

After July 1, bus transfers will not be accepted at rail stations, but rail stations will issue transfers for those who wish to continue by bus.

Those who choose to start practicing with Farecard on Monday can buy a Farecard from the vending machine and get change. The vendor accepts $1 and $5 bills, plus nickels, dimes, quarters and half-dollars.

After purchasing Farecard, the rider inserts Farecard into the entrance gate and retrives Farecard from the top of the gate. The gate opens. If Farecard's magnetically encoded tape tells the gate that there is less than 40 cents (the cost of the shortest ride) left on the tape, the gate will not open.

It is also impossible (Farecard's creators say) to hand Farecard back to a following relative or friend and have him use the same ticket. The entrance gate will not open a second time for the same Farecard until that Farecard has been through an exit gate.

After getting through the gate, the rider boards the train. At the end of the trip, the rider approaches the exit gate and inserts Farecard. Three things can happen, two of them good:

If th cost of the trip equaled the amount of money left in Farecard, the machine will eat Farecard and the exit gate will open.

If the cost of the trip was less than the amount of money left in Farecard, the machine will return Farecard and print the balance left in Farecard on its face. The gate will open.

If the cost of the trip is more than the amount left in Farecard, the machine will light up say, "Go To Addfare," and return Farecard. The gate will not open.

Addfare is a 4-by-8 foot machine standing inside the exit gate. The rider will insert the returned Farecard in Addfare. Addfare will figure out how much money the rider owes and will anounce same in a digital readout.

The rider inserts money into Addfare's coin and bill slots, retrieves Farecard and change (if any), returns to the exit gate, and tries again.

The Farecard system was adopted so that Metro could charge riders by the mile, instead of using a flat-fare system. Until July 1, flat fares of 40 cents during the off-peak and 55 cents during rushhour will be charged.

After July 1, fares will vary depending on time and distance, but the minimum for three miles will always be 40 cents. Higher rush-hour fares are charged between 6 a.m. and 9:30 a.m. and 3 p.m. and 6:30 p.m. Metro rail runs Monday through Friday from 6 a.m. to 8 p.m.