How about putting a SmartTrip card machine on the other side of the turnstiles or on the platform? That way, once you get to/near the platform and see how long until the next train, you know if you have some time to wait and you can refill your card while you wait for the train.
A lot of people rush through the refill process and then run through the turnstiles to the platform, fearing that the time they took to refill might've cost them the chance to catch the next train. There's usually no way of knowing until you get onto/near the platform if you have time to use the machine. Larger platforms (like Archives, Union Station, Anacostia, etc) could handle the space a machine takes up and it would cost very little just to relocate an existing one from the other area.