Simplify the following text for a Grade 10 native English-speaking person.
Original: The 1983 stock proved to be unreliable and troublesome in service , with single-leaf doors making passenger loading and unloading a slower process than on other stock with wider door openings .
Simplified: The 1983 trains proved to be unreliable in service , with their single doors making passenger loading and unloading slower than on other trains with wider doors .