Simplify the following text for a Grade 5 native English-speaking person.
Original: They were built at the BR Ashford Works over the period 1949 â `` 1952 and numbered 15211 â `` 15236 .
Simplified: The Class 12 locomotives were built at the BR Ashford Works over the period 1949 â `` 1952 . They were numbered 15211 â `` 15236 and became Class 12 .