Turning facilities of some kind were much more important in the steam era. As Bob said, it doesn't matter in the yards, where steam switchers would push or pull just like diesels but it mattered a great deal to road engines. A steam engine could run in reverse but not as fast as it could running forward and it was hard on the valve gear. In the days before the power reverse, just getting a road engine in reverse was a real chore. If I was planning a steam era layout, I'd include space for a wye ( or "Y", because of the shape of the track) at one end of the line and a roundhouse at the other. Railroads couldn't afford a roundhouse every place they needed to turn an engine so a wye track was used. The advantage of a wye for a prototype railroad is that it's relatively cheap. You just need four regular turnouts and a wye turnout. The disadvantage for modelers is they wye's take up a lot of space. You need a either a runaround track that will hold you longest theoretical train or the tail of the wye has to be long enough to hold that train. This is a good example of why planning your layout for operations before you build is so vital. If you don't plan a wye track, you'll find your steam engine trapped running in reverse, something real railroads always tried to avoid.
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