So, Danish, and Scandinavian, locomotives have small fire holes. You can not push a normal middle aged adult through the firedoor. How do you get in there for repairs? The answer is a special door through the ash pan. Here is the access portal through the ash pan of HHB 4.

And you can get a normal middle sized adult through that?

It may seem small, but it definitely does the trick! The most difficult part however is the removal of enough grate bars to then slip in above them. The first one requires a lot of finger strength to push the bottom of the bar upwards and come free of the ash that's managed to lock it in place. Don't ask me how I know....