Simplify the following text for a Grade 7 native English-speaking person.
Original: The OIR operated its own fleet of steam locomotives : 0-6-0T and 0-6-0ST locomotives built by Hunslet and 0-4-0ST 's built by Hudswell Clarke .
Simplified: The OIR operated its own fleet of steam locomotives : 0-6-0T , 0-6-0ST and 0-4-0ST 's .