Minecraft – Custom Minecraft 1.8 spawners

The reason you cannot spawn Minecarts like this is because of the way Minecart spawning work. As you know, a spawner only spawns items IF its spawning condition is correct.

For a Minecart (or boat etc.), the spawning requirement is that it is placed by a player activating it on a rail (of any sort). Because a spawner IS NOT a player, it cannot spawn it. Thus, the Minecart SaveGameID "MinecartRidable" is not usable because it is a placeholder of the original Minecart SaveGameID (which switches to MinecartRidable the moment it's placed/spawned), is "Minecart".

I've also tried it with tracks all around the Minecart spawner, which does not work because again, Minecarts must be placed (activated; right clicked on rails). Boats work because they behave like a spawn egg. Only, that it will only spawn in water.

EDIT: The reason it worked is because Boats and Minecarts do not have a spawning requirement in older versions... (I'm not sure which, but I think it's around 1.2 Release) Now, only items such as throwable potions and experience orbs do not have a spawn requirement.

You are standing 32 blocks away. Mobs spawn when you are at least 24 blocks away, but then despawn over time when they are 32 blocks or more away. The other thing is, you're standing at Y = 78. I'm willing to bet that you're within 128 blocks (spherical) of some caves somewhere, which are likely filling up with mobs (there are thousands of spawning spots in caves vs. likely a few hundred in your mob spawner), and they are keeping mobs from spawning in your trap. Between this and the fact that you are 32 blocks away means it is likely that the few mobs that are spawning could despawn before you have a chance to kill them. Finally, mobs freeze (don't move) when you are around 30 blocks away, so unless your pads are small, the mobs will stop walking before they walk off the edge into your water streams, assuming you're using that style of mob trap. This means they will stand up there till they despawn.

Recommendations: use small (3x3) spawning pads, with water streams in 2-block-deep trenches between them, and make it small enough that the mobs are quickly funneled into your trap before they have a chance to despawn. Etho has a very simple mob trap design that is easy to build, expandable, and efficient, and has remained so since he designed it over 2 years ago. I would stand somewhere at least 24 blocks away horizontally, around midway up the spawner and at least 128 blocks away from any caves that aren't lit up. Since mobs do walk around for a few seconds before freezing, with this mob farm, the mobs are funneled into your trap before they have much chance to despawn, so being over 32 blocks from some pads isn't a problem.