when i
/testfor @e[type=MinecartRideable,7,35,19,-5,35,19]in the commandblock its ignore the coordinates and it testfor all minecart in the loaded chunks
also i tried
/testfor @e[type=MinecartRideable,1,35,19,6]/testfor @e[type=MinecartRideable,x=1,y=35,z=19,r=6]/testfor @e[type=MinecartRideable,x=7,y=35,z=19,dx=-5,dy=35,dz=19]i hope i describe this well 🙂
The first two commands can't work, because the coordinates have to be the first arguments if you wan tto leave out the "x=". See http://minecraft.gamepedia.com/Commands#Target_selector_arguments