The railroad going up Mt. Tamalpais employed Shay locomotives as well as Heisler. I used to think they were all Shay but learned that the No. 9 that is being restored is a Heisler.
"The most significant design difference between Shays and Heislers was the arrangement of the driving cylinders. The Shays had either two or three driving cylinders mounted on the right hand side of the locomotive. Since the weight of the driving cylinders sat only on one side, the boiler was mounted offset to the left to provide a counterbalance. In the Heislers, the driving cylinders were located in the middle of the locomotive."[1][2]
[1] https://www.friendsofno9.org/post/mechanic-s-corner-geared-l...
> 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).