What are the giant openings atop certain GBRf trains

I was on a bridge close to St Denys station (Southampton) and noticed a GBRf train approaching (without any cargo attached). As it got nearer I noticed two large openings on the roof with a diameter of at least 1 meter, though it could definetley be even larger.

At first I thought that the train could have a turbine engine. But the only information I could fnid was about 1950s trains (eg the GT3). Alternativley I was thinking it couldve been turbocharged or something, but not sure why the openings were so large.

I tried looking through the GBRf website to see what type of train it was and the class 66 and class 59 looked similar from the front but the top was definetley different.

Anyone got a clue as to what these openings are for?