Hi Jayco-man
You can find this filter by following these steps:
1) Double click on the Trainz icon on your desktop
2) Click on Start Trainz
3) Click on Driver/Surveyor
4) Click on the 'Type' filter at the top of the screen, then select 'Multiplayer'
5) Click on the 'Source' filter at the top of the screen, then select 'All'.
You should then see any active multiplayer sessions listed.
Regards