Complex sentence: Two of the class, Nos.1625 and 1797, were converted to oil burning with more set to follow suit, but the project was abandoned and the two oil burners reverted to coal-firing before this could take place.
Split into simpler sentences: Two of the class, Nos.1625 and 1797, were converted to oil burning. Other members of the class were also designated for conversion, but the project was abandoned before this could take place.