Simplify the following text for a Grade 10 native English-speaking person.
Original: The line currently operates fully automated , trains running every four to ten minutes without a driver , and the carriages were changed from the existing M-Train rolling stock to match the recreational and adventurous nature of the 3.5-minute journey .
Simplified: The line currently operates two four-car fully-automated trains ( each car accommodating up to 180 passengers ) running every four to ten minutes . These carriages were converted from the existing rolling stock to suit the recreational and adventurous nature of the 3.5 minute journey .