Historic Semmering Railway is a stretch of 41-km between towns of Gloggnitz and Mürzzuschlag over Alpine terrain. It is best explored by car, which allows for stopping at strategic viewpoints to take pictures of its remarkable viaducts. I instead rode its entirety on a slow train, which is a middling experience, not specifically necessary to enjoy the most dramatic scenery.
The villages along the way are not especially remarkable, but there is some Alpine leisure architectural gems that can be found here or there (again, best explored via a car). I got off train at Semmering for about an hour and a half, which was beautiful in snow in December, but also practically deserted away from the skiing resort areas. There is not much to do in town if you are not skiing.
The town of Mürzzuschlag houses the railway museum, which I did not visit.