In 1.8.8 you could define where entity from spawner whould appear:

/setblock ~ ~2 ~ minecraft:mob_spawner 0 replace {SpawnCount:1,SpawnRange:1,Delay:-1,MinSpawnDelay:50,MaxSpawnDelay:50,MaxNearbyEntities:1,RequiredPlayerRange:-1,SpawnPotentials:[{Type:MinecartCommandBlock,Weight:1,Properties:{Pos:[10.5,87.0,10.5]}}]}

In this case, minecartCommandBlock appeared at 10,87,10 every time the Delay hit 0. It was easy to set activator rail to run the command in cart

When I rewrited this for 1.9 snapshots, the Pos[x,y,z] entitydata was ignored. Tried both ways in Entity tag and in Properties tag:

/setblock ~ ~2 ~ minecraft:mob_spawner 0 replace {SpawnCount:1,SpawnRange:1,Delay:-1,MinSpawnDelay:50,MaxSpawnDelay:50,MaxNearbyEntities:1,RequiredPlayerRange:-1,SpawnPotentials:[{Entity:{id:"MinecartCommandBlock",Command:"hi",Pos:[10.5,87.0,10.5]},Weight:1}]}

In this case, minecartCommandBlock appears near spawner, randomly, definitely not in specified position. Setting SpawnRange:0 is still random y+-1, so minecart can spawn in spawner, over it or under.
In 15w40b the second command was working, in 15w41a it stopped working. Save file in attachment to easy check when it works and when not. It can be opened in 1.8.8 and in snapshots of 1.9.