The Trace tool from the Editor toolbar cannot trace densified lines and curves. The tool appears to trace, allow a start point to be selected but does not create the trace line.
Starting with the initial release of ArcGIS Pro the technology behind the trace tool was significantly upgraded. As a result tracing on densified features is smooth and responsive in ArcGIS Pro. Unfortunately this upgrade cannot be included in the 10.x release due to technical limitations.
解决办法
Use generalize or simplify to reduce the number of vertices if a standard or higher license is available.
Zoom in and use the Trace tool at a scale less than 1:1000 and complete small sections using the arrow up/down left/right to move while using the Trace tool.