Simplify the following text for a Grade 14 native English-speaking person.
Original: The term train operating company ( often abbreviated to TOC ) is used in the United Kingdom to describe the various businesses operating passenger trains on the railway system of mainland Great Britain under the collective National Rail brand .
Simplified: The term train operating company ( often abbreviated to TOC ) is used in the United Kingdom to describe the various businesses operating passenger trains on the railway system of Great Britain under the collective National Rail brand .