i was trying to use command blocks to summon minecarts that had command blocks in to create custom mob spawners.
the minecarts ran on tracks underground.

Here are a couple of examples i tryed to use:

summon MinecartCommandBlock ~ ~ ~-1 {Command:"summon Slime ~ ~7 ~ {CustomName:"Forest Slime",Size:0,Riding:{id:Slime,CustomName:"Forest Slime",Size:1}}"}

summon MinecartCommandBlock ~ ~ ~-1 {Command:"summon Wolf ~ ~5 ~

"}

the command blocks do not summon minecarts with commands in unless you remove the custom names from the command.

my guess would be that the custom name speech marks conflict with the command ones.