Use red flags for all northbound trams and yellow flags for the southbound trams. This will avoid confusion as the yellow tram line was eliminated this year but northbound and southbound trams were available for boarding at the station across from the control tower. I overheard several conversations between drivers and passengers along these lines:
"Sir, where can I board the southbound tram?"
"Right here. It's the one with the red flag."
"You mean, like yours?"
"No, we're going north. The other red flag goes south."