Simplify the following text for a Grade 15 native English-speaking person.
Original: Although not entirely successful in their own right â `` they were seen as underpowered , unreliable , and ultimately not economically viable â `` the Blue Pullmans demonstrated the possibility of high-speed fixed-formation multiple-unit inter-city train services , and inspired the later development of the Inter City 125 , which resembles them in having an integral power car at each end of the train .
Simplified: Although not entirely successful in their own right , the Blue Pullmans demonstrated the possibility of high-speed fixed-formation multiple-unit Inter City train services and inspired the later development of the Inter City 125 .