Larry says:

Any old analog train engine can conceivably be converted if enough room can be found for a decoder.
If the loco is a poor runner with bad electrical contact characteristics to begin with, they won’t run well on DCC either.
Diesels tend to have enough room to install decoders (or enough room to mill their frames for one), and most steam will have room in their tenders. Newer locomotives are likely wired for DCC or were designed with ample room for decoders.