Simplify the following text for a Grade 11 native English-speaking person.
Original: The Advanced Passenger Train ( APT ) was an experimental tilting High Speed Train developed by British Rail during the 1970s and early 1980s , which did not enter regular service .
Simplified: The Advanced Passenger Train ( APT ) was an experimental tilting High Speed Train developed by British Rail during the 1970s and early 1980s .