Simplify the following text for a Grade 10 native English-speaking person.
Original: British Rail 's third design of standard carriage was designated ` Mark 3 ' ( also described as Mark III ) , and was developed primarily for the InterCity 125 High Speed Train ( HST ) .
Simplified: British Rail 's third fundamental design of carriage was designated Mark 3 , and was introduced in 1976 primarily for use on the High Speed Train ( HST ) .