Again, poking around the Atlas website https://shop.atlasrr.com/c-6-002.aspx, I don't see a combined UC+UL piece of track. You have to "build your own" using a combination of using a #6059 and the rails from #6097 (and possibly some short pieces depending on your track plan).
The other option is adapt to Gargraves or other track brand just for that one spot (example using #6096 Grargraves transition rail joiners).
or another way is tubular Lionel 6-65530 O Gauge Tubular UCS Remote Control Track and O TRANSITION JOINERS 6/BLISTER #6095 Note, one potential problem with tubular is the railhead height match between the taller tubular O section and then Atlas on a typical roadbed.