Possible Clues:

• 'Alongside something, it's all pearl (8)'
• Meeting's impossible in bars like this
• Lines that never the twain shall meet
• Alongside in time
• How some main streets run
• Going in the same direction, always the same distance apart
• Not meeting the standard on whole railway
• Continuously at equal distances apart
• Everywhere is equidistant and doesn't intersect
• Correspondence suggesting inability to make projected meeting