they never originally did. back in the 30's the converted an old Lumber transfer crane, to transfer cars. A really fasicinating process. The SG car would be rolled up to the crane and set the breaks. Then, the car was lifted off of it's standard gauge trucks, and those trucks were PUSHED away by employees. Next NG trucks were rolled up under the car, and lined up. Finally, The car was dropped onto the trucks, to continue it's way along the NG line. It's the same for converting back to standard gauge, only reversed. Interesting fact about it, The SG cars never very far on the southern portion due to tunnel clearances.
There is Dual gauge currently in existence, but it is an entirely different railroad, the Mount Union Connecting railroad. Back in 2004, Someone tried to get a freight service running to the industrial park, about a mile away from the yards. So they cleared the track, put in a state of the art dual gauge crossing with all the bells and gates, (one of the only ones in America if I am correct) and stablelized the track. Now we get to the part where they finally order a locomotive second hand, but the loco they ordered came DOA, with a lot of work needed to get it to run again. Also the Industrial park has not had any tenants show more than interest in it.