Model locomotive chinmeys used to be supplied "blind", or blanked off at the top. The exceptions were those with a screw in the bottom to hold the body to the chassis. You still see this in some whitemetal castings but not often.
This might be a diesel but it still has something that looks a bit chimneyish for the exhuast fumes to escape. Quite a chunky pipe this, I don't know if it needs to be or is just the whim of the designers. Whatever, the top was solid and since we see model railway from above most of the time, I didn't feel this was satisfactory.
With some trepidation, I started to drill the metal. To my surprise it was quite soft. My scriber could be pushed into the centre of the soon-to-be hole and this guided a 1mm drill. Following up with a wider bit took some time but was still accomplished with hand power rather than electricity.
Emboldened, I thought I'd drill out the front of the blank airhorn. That wasn't so clever as the plastic body twisted as soon as the drill bit. I pondered the problem and then made it worse by snapping the thing in two.
Luckily I have a box of spare bits and this included a packet of rather nice brass airhorns. Drilling the the bracket I quickly put one in place with a touch of superglue. To be honest, I should have done this in the first place as it looks a lot better.