One Way Signs

[BramNieuwpoort]

Hello,
I've started playing TTD not to long ago, and I have just one little question.
How can I make one way signs, because everytime I make a sign it's two ways.
It's for the railroad tracks.

[Yexo]

Select the signal tool and click again on the tile with signals.

[BramNieuwpoort]

Thank you