Simplify the following text for a Grade 7 native English-speaking person.
Original: The British Rail Class 317 alternating current electric multiple units ( EMUs ) were built by BREL York in two batches , from 1981-82 and 1985-87 .
Simplified: The British Rail Class 317 electric multiple units were built by BREL at York Works in two batches , from 1981-82 and 1985-87 .