A solid resource for track centers can be found on line at the NMRA site. A number of Standards and Recommended Practices are important to consider as you plan a layout. Here are a few.

S-7: Clearances - A key standard addressing distances between tangent track centers and clearances with lineside obstructions.

S-8: Track Centers - A standard for distances for clearance for various curved track, and noted for several modeling scales.

RP-11: Curvature and Rolling Stock - This RP addresses minimum radius curves for rolling stock of various lengths, and over several modeling scales.

These are the building blocks for modelers to create layouts for equipment to operate without coming into contact with locos or rolling stock on adjacent tracks, or with lineside structures or scenery. This data is the starting point for modelers to consider as they develop their layouts.

Now, as many turn to prototype modeling, following the data of the specific prototype during the era you model becomes another layer of information that can be used in your railroad modeling. As an example, I model a prototype that was originally built as a narrow gauge in an urban area, and my focus is 1926. I have chosen to follow prototype clearances where I can. Tangent track centers for double track and passing sidings are on 13 foot centers. Curved track is a little different as my minimum mainline radius is 24 inches. Concurrent curved track centers are more generous, but easements into tangents at each end of the curves bring the track back to the 13 foot center-to-center distance. Tangent track centers for industiral sidings adjacent to mainline tracks are on 16 foot centers. These track center distances were found in a 1921 Maintenance of Way Cyclopedia, which is also online through Google Books.

Additionally, vertical clearances are tighter than the NMRA standards, especially as my efforts are focused on the standards of 1926. Obviously, with this focus on prototype standards of 1926, I will not be able to operate 1990's style double-stacks. Over time, railroads have increased the distance between running tracks in many areas and also increased vertical clearances to operate larger equipment. When deciding to follow a prototype more closely, you will make decisions based upon what was used on the prototype you are modelling.

Our hobby is all about choices. The NMRA details on track centers, curvature and clearances offers great data to help anyone plan a new layout. If you are more focussed on a specific time and place, using prototype data is another layer of information to consider as you plan and build.

Eric