Agree with Rob, no best location. My general rule of thumb is to limit the number of transitions as much as possible, but use them were needed.
Rio Grande. The Action Road - Focus 1977-1983
I would make the whole crossing code 83 and have trasnsitions from the code 70 track before the crossing.
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It will look fine, and no one will notice once the track is painted and ballasted.
-Kevin
Living the dream.
I don't think there's a "best" place. Wherever you can make the transition smoothly will work. Facing the same situation, I tend to not make such transitions between turnouts on a crossover, as it's often more difficult to get the rail alignment just right. I've done it and it has worked reliably, so both options are definitely do-able.
Rob Spangler
I have a situation where I have a crossover between a Code 83 mainline and a Code 70 siding. Where is the best place to do the transition between rail? Between the two switches or lay the entire crossing as a code 83 and make the transition off both ends of the switch on the siding.Logically, it would seem between the switches but the local opinions are varied.This is an interesting situation in RL. During heavy citrus traffic time precooled reefers would be dropped off on one side of the crossover and then switched to their destination. The other half of the siding would be used to set out the loads. Interestingly enough there is another siding on the other side of the main with leads to packing houses and team tracks. The place was small but busy.