The most significant design difference between Shays and Heislers was the arrangement of the driving cylinders.

Another very important difference (and one that makes Heislers superior to Shays) is that the driveshaft is centered on the loco frame.

That means the driveshaft doesn't need expansion joints, which tended to get jammed under high torque and prevent the trucks from pivoting under the local (thus splitting the rails).