The NMRA standard is always a good way to keep form having any problems but what really determines the space you need between tracks on curves more so then the radius of the curve is the length of the rolling stock and locomotive in some cases. For example an 85' heavy weight passenger car will have more swing to it then lets say a 40' box car. another factor is the type of coupler mounting Talgo or truck mounted couplers although not prototypical will have less swing then body mounted couplers.In regard to locomotive longer engines such as big articulates like Big Boys or Mallets even though because of their articulation are able to negotiate tighter curves the trade off is more "cab overhang" I call it. So care needs to be taken how close you place things like sign and telephone poles etc. in relation to the tracks. On something like a 4-8-4 you will have much less cab over hang but require a larger radius curve .This isn't much of an issue with diesel locomotives with the exception of the curve radius when it comes to big 6 axle road engines. Over hang is never an issue with a diesel.
If you don't own one already pick yourself up an NMRA gage , this will definitely be helpful with spacing track as well as numerous other tasks.
I asked the same question a few weeks ago, and I think this is the most popular question on the forums. I followed my fellow modelers advice and used 2 1/2" on the main line and 2" in the yards. It worked out great after the ballast was spread and is very prototypical looking.
The NMRA track-to-track standards are a good place to start
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Welcome to the forums.
2" on a straight is "normal" though you will find variations. On curves it depends on how sharp your curves are and how long your rolling stock is. Yard spacing can be closer, BUT, if you have to rerail and fiddle cars, put a few pieces of track on the table and see how big your fingers are.
Good luck,
I think usually 2" is a good general measurement.
Modeling New Haven despite what his user name says...
I am palnning a very simple HO shelf layout around my rec room. Two parallel tracks only. How close together (from centre of track) should the tracks be? I had it written down but can't find my notes. Thanks.