AEC symbology should be able to represented the same as you see it on ArcGIS Pro.
AEC symbology for specialty like rails has a specific symbology that is only available inside ArcGIS Pro. As soon as upload to AGOL, the symbology cannot be replicated on the cloud.
What you see on ArcGIS Pro should be what you get on AGOL.
ESRI Support Case number: 03954019