The portals do not retain the vehicle names or running numbers or the train priority, nor is it possible to set these using a portal.
However, there is a driver command available on the DLS called "priorityz" by Smileyman (kuid:66277:80003) that allows the driver to set the priority of the train.
To use this, add it to your drivers command list immeadiately after the drive to command that sent him to the portal. This will then be the first command in the list when the train leaves a portal. You can also use this command to set the priority of the train if is created by the portal by including it in the drivers command for the train.
Regards
Maggs