I recently started to build my layout with code 100 flex track, I have just aquired a whole lot of code 83 flex track from another layout I bought, Can I mix an match these codes and how would I go about it.
Code 100 and Code 83 track can be joined end-to-end with no problems. If both are Atlas flex, you won't even have to shim under the Code 83 ties to keep the railheads at the same level.
Once again, using Atlas flex, I simply joined the two sizes with standard rail joiners, with 0.017 inch slivers of plastic under the code 83 rails. Others have suggested more complex arrangements, including special 'stepped' rail joiners.
If you have already laid a fair amount of Code 100, you can use the Code 83 for secondary trackage. Or you can lay your visible track with Code 83 (and smaller) rail, saving your Code 100 for hidden track. A third possibility is to lay code 83 flex and put some grunge-painted Code 100 stick rail along the right-of-way as preparation for a line upgrade. My modeling includes both of the above - code 100 in the netherworld, where its appearance isn't critical, and code 83 (on concrete ties) on visible track scheduled for 'upgrading.' The 'stick' Code 100 is some ancient brass rail recently stripped from fiber tie strip - the original Atlas flex track.
Chuck (Modeling Central Japan in September, 1964 - with Atlas flex and hand-built specialwork)
At least at one time Walthers made a code 100 to code 83 transition track, about a 4" straight that was 83 on one side and 100 on the other. You could check at the LHS.
Railroads really did use different sizes of rail, but generally for specific reasons. For example, a mainline would often have heavier rail than a siding or spur track coming off from it. If you're going to use both code 100 and 83 you might want to do something like that, rather than just have the layout be code 100 until at some point it changes for no apparent reason to lighter code 83 track.
Atlas also makes inexpensive code 100 to code 83 transition rail joiners. Micro Engineering makes a code 100 to code 83 transition insulated (plastic) rail joiner. And Walthers continues to offer the transition length of track. This is a very cleverly designed item that really makes the transition almost impossible to detect.
You do see transitions to different rail sizes on a main line, such as where jointed rail meets welded. I have seen some crossings with larger rail than the main, perhaps in preparation for an upgrade to the main.
Another thought is to retain the larger track for the mainline but then do what the prototype did: smaller rail for yards and sidings, smaller yet (code 70 or Peco's code 75) for very lightly used sidings and industrial tracks. While Code 83 and 70 would be relatively large for these purposes (code 70 and code 55 would be closer to scale), this would preserve the relative sizes of rail for the different uses.
Dave Nelson
dknelson You do see transitions to different rail sizes on a main line, such as where jointed rail meets welded. I have seen some crossings with larger rail than the main, perhaps in preparation for an upgrade to the main. ---- snip ---- Dave Nelson
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more likely, in expectation that the heavier rail would last longer in a place where changing it out would be a major inconvenience to the public.
I have also seen specialwork in places with lots of traffic that was built with rail and castings much taller than the 'plain Jane' track outside the borders of the interlocking plant - again, the intention is to have it last longer in service. That's a useful prototype for someone who wants to recycle Code 100 turnouts on a layout that is mostly Code 83.
Chuck (Modeling Central Japan in September, 1964)
All of my hidden staging yards are Atlas code 100 flex and turnouts as mentioned the Walthers transition tracks work great. They are so undetectable you have to watch you don't try and put the code 83 side on the code 100 track, ask me how I know this.........lol My motto: If you got it use it you'll always find a way to make it work.