Command block minecarts have the following data walker:
This will never run, because it is comparing a String constant with a resource location. (This issue was introduced in 18w01a). This is not an issue for spawner minecarts, which use if (Objects.equals("minecraft:spawner_minecart", (new ResourceLocation(s)).toString())).
The code also seems to be using old (pre-1.11) IDs, which is incorrect; see