Related Clues:

• London's underground, informally
• Train needs energy except for reversing
• ___ steak (hot dog)
• Caulk container
• Project Mercury food source
• Brit's subway
• Bike tire's innards
• Inner ___ (flotation device)
• Cable box display
• Rotavate earth, except underground
• Phish "First ___"
• Concealed in situ beside foreign transportation system
• Transport system, however revolutionary, ultimately late
• Cylindrical hollow conduit
• Charmin roll's center