Anything goes, as long as you don't loop it back on itself (in your existing config - another cable running from the DB150 to the Zephyr would be a no-no). On more complex setups it's common to have the command station and all the boosters daisy chain directly to each other with no UP5's, UR radio panels, or other accessories in between, but that's mainly to keep the whole system from shutting down in the event one of the UP5 jacks goes bad from frequent plugging and unplugging of throttles and has a bent pin short out the loconet. FOr REALLY large layouts, they came up with the Loconet Repeater LNRP which can actively isolate a faulty segment from the rest of the system. All far more than you need at this point.
Modeling the Reading Railroad in the 1950's
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The loconet doesn't work in a particular order, it just sees what is on the network much like a computer network. Putting the third DS64 in should work just fine. As long as everything can see the command station and vise versa, it will all work correctly.
currently my digitrax loconet wiring is as follows; it starts at my booster (db150) from there it daisy chains to a ds64 to a up5 to a ds64 and dead ends at second booster for my turnouts ( dcs50 zephyr), in an a to b oreintation. my question is that i want to add a third ds64 before my command station and run my loconet wire from the new ds64 to the command station (a to b). Just wondering if this would work and still communicate properly??