That question interested me enough to provoke a trip to the basement to test my 726 Berkshire. I put it on straight track, coupled the tender and rolled it around the curves. Surprisingly, it rolled very well around the curves, but with a large overhang. I had thought maybe the tender would hit the locomotive, but no problem there as the cab roof is higher than the tender. It looks ridiculous !!
The overhang is impressive - 2 1/4" on the outside for the cab roof and 1 9/16 on the inside for the the main crank pin screw - from the third rail. The offset between the centers of cab and tender is about 1 1/2".