Which is the high pressure (supply) line at the fuel rail?

The return line is the one that is leaving the pressure regulator so the other line coming into the fuel rail is the supply line.

The regulator is on the return side of the system, and it controls the pressure by allowing the proper amount of fuel back to the tank, so they are after the injectors.

So, this means then line that is connected to the regulator is the return line.

If you are still not sure, then just connect them to a rubber line and run them to a bucket and the one that squirts fuel is the pressure line.

Please let us know what other questions you have.

Thanks