steps for this to work:
1.) get a command block /give @p command_block
2.) go about 2 blocks high
3.) do /setblock ~ ~ ~ stone (select whatever block you like, you can break it later!
4.) place a command block under it
5.) insert the command below
6.) set the command to repeat and always active
7.) you should see a creeper line coming through!
to stop the conga line, break the stone and command block!