2 30 ton tanks that fit on universal flatcars. When we went with the longer barrel 76mm gun one of the requirements was the ability to be able to stow the tank with its turret facing the rear with no over hang. So even with a long barrel 2 M4 Mediums could be put on a flatcar. read more
The reason why some tanks didn't have an over hanging gun is simple: in the early days of tanks any tank likely to be engaged (actually that should read "expected to be engaged"[1]) could be handled with the typical anti-tank weapons of the day: small calibre (37-50 mm) and relatively low muzzle velocities. read more