Often wondered. Wouldn't it be cheaper and less hassle to simply park the doomed train in an isolated siding, issue the local youths with sledgehammers and then send along an engineering train of flatbed wagons to pick up the pieces. Sorted.
It would actually make the process a whole lot more expensive for various reasons. Mainly, as buildings/trains/cars etc are ripped apart, materials and components are sorted out. If you scrapped the whole thing as a lump, you would get mixed material cost back, rather than getting a proper total for copper, aluminium etc
It's an interesting topic you bring up and one that is rarely publically explained in an official capacity if ever, but virtually every component in scrapped trains is stripped or cut up to be recycled or put to use in some way shape or form.
Contrary to popular myth, this is skilled work which is why it is exclusively the domain of specialist firms such as C F Booth and has been for some time especially now that there is greater awareness in the industry about managing the disposal of materials used in the past that have since been found to be environmental hazards asbestos being just one.