StartLine.cs
using Sandbox;

public sealed class Startline : Component
{
    public void PlayerEntered( GameObject other )
    {
        var redLightPlayer = other.Components.Get<RedLightPlayer>();

        if ( redLightPlayer == null )
            return;

        redLightPlayer.State = RedLightPlayer.PlayerState.Playing;

        Log.Info( "player Start" );
    }
}