For most layouts, I would definitely consider adding a model railroad turntable, or at least some way of turning trains around. A simple wye is an option as noted on the Track Planning page, but not as dramatic in presentation and operation as a turntable. It may not be appropriate or necessary if you just want to build a small switching layout or shelf layout, but for most other operational layouts, it really adds a great scene and much more functionality.
The model railroad turntable also allows for a generous display of your model train locomotives along with a model railroad roundhouse and connections to a freight yard where you can show off your weathered freight cars. This creates a large wonderful scene that can become very detailed and can depict a hotbed of operational activity for your layout.
If you are a scratch-building master, you can look at photographs, learn about model railroad turntable operations and build one yourself, which would be time-consuming, but not that difficult. The building process would be enjoyable, and when it’s finished, it’s something you could be quite proud of – like scratch-building your own model railroad buildings or hand-laying track. You can win Achievement Awards from the NMRA for doing things like that, which could eventually lead you to become a Master Model Railroader.
If scratch-building is not something you have the time or inclination for, you can purchase a √ model railroad turntable kit along with a √ roundhouse and all the accessories you need to make a great operating scene and a center of activity on your layout.
There are several to choose from. Some are open pit and some are flat-surfaced. Some are manually operated and some are motorized. Most of the manually operated turntables can be converted to motorized later by adding a small motor made by the same manufacturer.
I would invest in a good model railroad turnout in the beginning, because it would be difficult to replace later after it has been installed in your layout.
I used a motorized √ Atlas flat-surfaced turntable (motor unit is purchased separately) in the scene pictured above which has served me well over the years operationally, although I think if I were to do it over again, I would go with an open pit model, which I think is more realistic. Here are a few for consideration...
Installation of a model railroad turntable is not always the easiest thing in the world. Make sure you read the instructions that come with your kit carefully because they are different for each one. Here is a video teaching the installation of the Walthers HO Scale 130' DCC turntable...
A roundhouse for locomotive storage and repair can also be purchased in various sizes. You could consider a 3-stall roundhouse and add a second or third to it later as your layout grows. Or perhaps just go with the 6-stall roundhouse right off the bat which is what I did.
Consider also adding a Rail Shop for locomotive or railcar repair and a fueling facility. If you are modeling in the steam era, you will also need a coal bin and a water tower. If you use diesel locomotives, then perhaps have a fueling station close by.
Add lights, a welding arc display from inside one of the buildings, train people, a junkyard and maybe a fence to hide it from view of the local neighborhood.
This just may end up being one of the most interesting scenes on your layout!