I must say that I'm surprised that this appears to be the first case involving gunpowder. It is easy to procure in quantity, and although a low explosive, a couple of pounds surrounded by shrapnel detonating on a tube train would kill a lot of people.
The different effects in a firework are due to grain size, the stuff in the fuses is large grain, the stuff that makes the bang is small grain. It isn't difficult to turn it all into small grain. That said, caution is required, otherwise you remove your eyebrows and the front of your hair, and blow out the garage windows.
We used to use it for blowing out tree stumps/roots. Because of the organic nature of the material, gelignite doesn't work. We sometimes used gunpowder and Ammonal
Ammonal - Wikipedia together.
Ammonal came as pellets, and we mainly used it where the rock had fissures. In that case the fissures acted as vents, and reduced the effect of the gelignite.